Many people that get into Warhammer Fantasy begin their GW hobby with Warhammer 40k. Or vice-versa. A lot of this overlap tends to begin with the overall judgement that they are similar games. To a degree, they are. But most of these similarities are in appearances only. The only real rules that overlap both systems are the To Hit and To Wound damage charts.
Movement
Warhammer 40k is a skirmish game. Units are loosely bunched together, can move in any direction and generally have fun in terrain. Movement values are generic and with the exception of a few unit types, are the same throughout all the armies. There is also only a single type of movement in the movement phase, although the fact that Warhammer 40k is a skirmish game is the main reason for this.
Warhammer Fantasy is a unit game. Some units operate in a skirmish formation, but most will be bunched together, base-to-base, in groups of 10, 15, 20, or even more models to a unit. These units have facings, which are front, back and flanks. A unit can only move to the front, unless it spends part of it's movement allowance turning to face a new direction. Different units in different armies also have different movement values. A unit of core troops in one army might have a movement value of 4, while a unit of core troops in another one might have a movement value of 5 or 6. Warhammer Fantasy also has several different types of movement that can happen. Which includes movement directed by the player, which can consist of normal movement, marching, or charging into combat. There is also compulsory movement, where units more often move about on their own, usually in a direction either toward the table edge or in a completely random direction.
Shooting
Shooting in Warhammer 40k and shooting in Warhammer Fantasy follow the same "to hit" charts. However, in Warhammer 40k, the roll the chart says you need to hit is all you have to worry about. In Warhammer Fantasy, the roll the chart says you need to hit can be moved up or down depending on modifiers such as long range, shooting at lone characters and skirmishers, or shooting at a large target. In addition, shooting in Warhammer 40k is much more important, since it's a skirmish game it allows the whole unit to shoot; while Warhammer Fantasy with it's block unit structure is much more restrictive in which models can shoot. Missile fire is also much rarer in Fantasy. It's entirely possible to field a whole army without a single ranged attack, while in Warhammer 40k, there is no such thing.
Close Combat
Getting into close combat is different in both games. In Warhammer Fantasy, charging into combat is done in the movement phase and is reliant on which direction the unit is facing and how far they can move, while in Warhammer 40k, charging into combat is done at the beginning of the assault phase and is done at a generic rate similar to how movement is done. The skirmishing formations of Warhammer 40k also mean that direction is irrelevant when charging, making it tricky to get around units without either charging them or being charged. Close Combat in both Warhammer 40k and Warhammer Fantasy use the same "To Hit" and "To Wound" tables. Neither game includes general modifiers to the "To Hit" table. Although Warhammer 40k does have a special rule that affects the "To Hit" roll, and Warhammer Fantasy has magical items which can add bonuses or penalties. Rolling "To Wound" likewise has several ways to affect the roll in both games without having any general rules. The rules for armour saves, however, is fairly different between them. In Warhammer 40k, armour saves are common and never modified, they are commonly denied entirely instead. In Warhammer Fantasy though, while armour saves are sometimes denied entirely, they are also commonly modified due to high strength attacks. Combat resolution is also different. In Warhammer 40k, combats are won or lost based solely on the number of wounds inflicted. While in Warhammer Fantasy there is static combat resolution, which makes it entirely possible to win a combat without even inflicting a single wound on the opponent. The form of the units also make a significant difference in combat. The skirmishing formation in Warhammer 40k allows all models in the unit to get into base contact to fight, allowing for a lot more attacks, while the unit block formation of Warhammer Fantasy vastly restricts the number of models in contact and fighting.
Psychology
Psychology is a very important aspect of Warhammer Fantasy as opposed to Warhammer 40k. While Warhammer 40k does have morale checks for certain situations, Warhammer Fantasy has panic tests, break tests, Hatred, Frenzy and a myrad of other rules that count as psychology that can drastically affect the flow of the game and provide bonuses and penalties in close combat.
While not the single most important aspect of Fantasy, games can easily be won or lost based on psychology tests. An unprepared general can quickly find his army fleeing the battlefield with no hope of recovering or being led around by the nose and unable to perform their duties. Being prepared is not as easy as it may sound though, and sometimes the dice will turn against you regardless of the steps you take to ensure success. What follows is a list of psychological terms in Warhammer Fantasy, how to be ready to deal with them, and also how to use them against an unwary opponent.
Panic
Panic is one of the most commonly occuring psychology tests a general can face during a game. It can appear in many forms, including from shooting casualties and from units being broken, destroyed or fleeing too close to other units. Quite a few armies have Ld values high enough to ignore these tests the vast majority of the time and some armies have rules which allow them to ignore the tests completely in certain situations. Still, a bad roll can break up an otherwise imperturbable battle-line and create gaps a clever opponent could quickly exploit.
There are ways to protect yourself from these situations though. The first good way to protect yourself is high numbers of troops. The more models in a unit, the more have to die in a single round before the test happens and the harder it becomes for an opponent to reach that number. Another good tactic is to include a character in the unit. He can give the unit his Ld, which, although it might not be a big difference, can sometimes be the difference between the unit fleeing at the worst possible moment and holding steady. Taking a musician is also a good idea, it's a plan for the worst, since he won't actually help prevent you from failing the test, but he does give you a bonus to rallying should you fail. There are also a multitude of magic items that either make you immune to panic or allow you to rally without the worry about a test, any of which are good if you think you're going to be taking a lot of tests.
A clever general can also use Panic to his advantage. One way to do this is to focus your shooting. A panic test kicks in once a quarter of the unit has fallen so it is a good idea to focus enough shooting onto one unit to force the test before moving on to shooting other targets. Another tactic is if you find yourself facing a fairly uniform battleline with a powerful unit. If you know you can break any of the units in the line on the charge, then you should probably aim for the center, since once the unit is broken, the units on both sides will then be taking panic tests and if you get lucky what was once an intimidating shield wall turns into an unruly mass of fleeing models.
Fear
Fear is the most common test you'll see in Warhammer Fantasy. Every army has at least one unit that causes Fear or they have access to a magic banner or item that makes the unit carrying it cause Fear. Fear is also one of the less effective psychology tests, since a failed test doesn't necessarily mean your unit is running screaming from the field.
Fear is one of the easier psychology tests to prepare for. The easiest way to be ready is to face Fear with Fear. Units that cause Fear, aren't affected by Fear, thereby negating any worries about failing the test. Whether the unit itself actually causes Fear or carries a magic banner or item that gives it to them doesn't matter, as long as they cause Fear, they won't fear it. Large unit sizes will also help out here, since as long as your Fear-causing opponent's don't outnumber you, even a failed test won't be as much to worry about as it could be. A failed test will still make things harder, but not having to worry about rallying is much better then the alternative. Here also, things that help you rally will be good to prepare for the worst, since the fewer tests you have to take, the fewer you have to worry about failing.
Fear can be a very potent tool for those generals that decide to use it. Some entire armies, such as Undead and Daemons actually lack units that don't cause Fear. Undead can be especially dangerous when it comes to Fear, since they are likely to have large units to guarantee outnumber due to the cheap cost of their models. Fear is also a good way to protect a smaller, more valuable unit, since whether they outnumber or not, a failed fear test on a charge attempt means no charge, allowing your smaller unit to get the charge instead or maneuver away and out of charge range for the next turn.
Terror
Terror is an upgraded version of Fear, you don't have to outnumber, and a failed test means the same thing whether you're trying to charge, being charged, or just nearby. Fortunately though, creatures that cause Terror are few and far between and are usually expensive enough that most general's won't just throw them out into the open where they can quickly get shot down. Also keep in mind that once your unit has taken a Terror test, pass or fail, it only counts as Fear for the rest of the game. After all, after you've been scared witless the first time, you don't have the brains left to have it happen again.
Fear is your friend here. Units that cause Fear only test for Fear against something that causes Terror against everything else. This makes it much easier to take since almost never will you be outnumbered by something that causes Terror. Outside of Fear though, your best option is to include characters to raise the testing Ld of your units or to prepare for the worst and have several methods available to boost your chances of rallying. The very best option though, is to never take the test. Most things that cause Terror, you can identify as such fairly quickly and focus enough firepower on to take down long before it gets close enough to terrify you.
Using Terror is a bit more difficult. Since it's going to be fairly obvious when something causes it and it becomes a fire magnet. Dragons ? Meet Cannonballs. Still, the best way to get the most out of a Terror causing unit is to get it up close as fast as possible to as many units as possible. Since the more tests you make your opponent take, the more likely he is to fail one. For this purpose, creatures that fly are the best. Move up behind terrain to protect him from being shot and then on the next turn drop down behind his line in Terror range of as many units as possible. This also works well because the initial reaction to Terror is to flee from the creature causing it, meaning if they fail a test caused by a creature in their rear, they're fleeing out toward the middle of the board and hopefully into charge range of one of your other units.
Magic
Magic is a purely Warhammer Fantasy aspect. There are some correlations between Magic and Psychic abilities in Warhammer 40k, but these are more aesthetics then anything else. Psychic abilities in Warhammer 40k are generally used during the phase in which the ability's effect is most simliar to other activities. Abilities that work as shooting attacks are done in the shooting phase, ones that move the model are used in the movement phase. Magic in Warhammer Fantasy gets it's own phase during which all spells are played out.
So really, while Warhammer Fantasy and Warhammer 40k share a name, company and a few generic tables there is very little to compare them to each other with ruleswise. So while a lot of people will look at them and just assume they're fairly similar games, it's really comparing Apples to Oranges. They're both fruit, with colors that are close, but that's really about all you can compare between them. Warhammer Fantasy and Warhammer 40k are two seperate games, requiring knowledge of different rulesets and requiring different tactics and strategies to win and they deserve to be treated as such.