...you are in for some looooong reading my friend. I'll give a summary that includes some of the stuff Outcast said.
The Tau is the newest actual 'race' in the 40k world. The Imperium discovered them around 5000 years ago, making them a very young race fluffwise as well. Inhabiting the Eastern Fringes of the galaxy, the Tau are a extremely technologically advanced race that focuses on ranged combat. The Tau are small and somewhat frail looking, with grayish/blue skin. They are a relatively peaceful race, their main goal in the 40k world being to unite all the galaxy in the name of their empire or the Greater Good (a change in the 40k cycle, having a peaceful race instead of warlike aliens). However, not many other races want to join this so called Greater Good, thus creating conflicts that the Tau counter with some of the best weaponry to exist too date. The Tau originally were a savanna dwelling race, but through sudden changes in climate, they adapted suddenly and drastically, making them more intelligent and resourceful. These conditions made the Tau sudden burst upwards in the technological world.
The Tau are divided into four main castes that are based on the elements. The Fire Caste are the warriors in the Tau society, being hunters in the ancient Tau history. The Water Caste are politicians, bureaucrats and diplomats. The Air Caste are pilots and messengers because of the fact that they used to have wings a long time ago. Finally, the Earth Caste contains the labourers, scientists and every-day specialists, making clothing, research weapons etc. There is also a smaller and much more mysterious fifth caste known as the Ethereals, who long ago united the Tau from their civil warring ways, for they have an uncanny ability to calm and to relax the Tau, making them inspiring, reknown and highly respected by the rest of the Tau civilization.
Tau have very advanced and uniform ways of waging war. The basic trooper, the Fire Warrior, is not special himself, but is powerful non the less with the aid of Tau weaponry. The pulse rifle is a frightening weapon that is powerful and long ranged, making even the most basic Tau trooper a formidable shooter. Higher ranking Tau are allowed battle suits, such as the Stealth Suit, which allows the wearer too be hidden at all times, making them hard to target. Equipped with Burst Cannons they are infiltraters that destroy enemies by jumping out with their jump packs and unloading a hail of shots. The Crisis Suit is the main powerhouse suit in the Tau army, having a large quanitity of weapons too choose from and allows the wearer enhanced strength and toughness along with the jetpack described above. There are also fearsome battle tanks in the form of Hammerhead Gunships with insanely long range and devasting power. Broadsides are a variation of the Crisis Suit with twin linked Railguns (which are found on Hammerheads) on their shoulders. Finally, Pathfinders are scouts that lock targets for the rest of the army to decimate.
Due too their dislike for close combat and weak bodies, the Tau have the Kroot, a mercenary race that they saved from Orks, due the close combat fighting for them. The Kroot are a savage race that feasts on the remains of the defeated but are good fighters and are equipped with Tau modified guns that make them good riflemen as well. Drones are also avaiable too the Tau as fire support of protection. Lastly, the new Tau codex will be releasing Vespids, an alienoid race that resemble blue flying Tyrani/Wasp things which the Tau also have as allies.
Hope this helped, and I hope this makes you rethink your thoughts on the Tau being knockoffs, they're totally original and different from the rest of the 40k world. Enjoy!