Not quite sure what you mean.. in game terms? Morally, as depicted in the background?
The thing about 40k is that there is no one 'good' army- all have strengths and weaknesses, it all depends on your style of play. For example, you have Space Marines, who are good at everything but excel at nothing- a jack of all trades. There is the Tau, really good at shooting but dire at close combat, and the Tyranids, who are the opposite.
Fluff-wise, there is no good side- every side is out for their own interests.
-The Imperium of Man, seen by a lot of newbies as the 'good guys,' are the most fascist, xenophobic regime in the history of mankind.
-The Eldar are a cruel, dying race, who care little about the other races (for example, it was the eldar that manipulated Ghazgull Thraka into launching his war against the Imperium at Armageddon).
-Orks live to fight, fighting each other if there's no-one else.
-Tyranids are just completely alien.
-Chaos are evil, having given in to their base desires, every man for himself.
-Tau can come across as being good, but they are only interested in themselves.
-Necrons want to kill every living thing in the galaxy.
Hope this helps, go to
www.portent.net or
www.dakkadakka.com, the real fluff-buffs can answer any other questions you may have
