Hey Mak, glad your looking into Warmachine, hopefully I can answer your questions.
1. Theres really 4 types of units in Warmachine, Warjacks, Warcasters, Troops and Solos. The quickstart rules don't deal with Troops and Solos though, as the starter sets only come with Warjacks and Warcasters. The rules are pretty easy to learn for Solos and Troops, only real differance is the addition of leadership checks and the whatnot.
2. Heres the lowdown on the 4 factions...
Cryx- These guys are the underhanded, crafty, speedy bastards that just piss everyone off. The units are very low armoured, very high defense, and VERY high speed. They also possess the most potent offensive magic in the game, since every single one of their "Deathchickens"(nickname for their light warjacks) carry an Arc Node that lets the Warcaster channel through them. Not many ranged units, most of their units relie on melee or nasty tricks that are even worse.
Menoth- The Masters of Synergy. At first glance you might suspect Menoth to be weak. Many of their units don't apear to be quite as strong as other factions units, but when you start putting them together, things get brutal fast. They are kind of the "medium" army in Warmachine, a balance between Magic, Speed and Armour. They have a good assortment of ranged and melee units.
Khador- Brutatlity in a red shell. Khador dominates the upclose and personal battle. Tough, strong and sometimes painfully slow, Khador can take a beating and than beat the crap out of whatever tried to blow them up. They have the worst offensive magic in the game, due to their complete lack of Arc Nodes, but they have the best "army supportive" magic, and their warcasters can often negate the slowness of the army with well placed spells that increase speed or otherwise agment their army. Khador has little in the way of ranged attacks, but has some of the best melee in the game.
Cygnar- The Progressors- Cygnar is the forfront of technology and magic, and has some of the best ranged attacks in the game. Their Warcasters have very good offensive and utility magic that can easily turn the tide of the game. They are a lighter armoured, faster faction, but not as fast or as lightly armoured as cryx. Like Cryx, Cygnar Warcasters piss people off by how much they can manipulate the other persons army, and can shred an opponents army before it even reaches them.
::Edit:: You beat me to the punch Cailet
You probably explained things more clearly though - Cailet.