The Decepticons are one of the primary factions in the Transformers mythos. They are typically concerned with such things as galactic conquest, defeating the Autobots, amassing large quantities of energon, developing powerful weaponry, and beating people up. Not necessarily in that order.
Unlike the Autobots, whose leader is a Prime bearing a Matrix of Leadership, the Decepticons are led by the most powerful of their ranks. This tends to cause some conflict, given how everyone thinks that they're the most powerful. Also, the Decepticons are not exactly the most compassionate beings in the universe, but not all fight for greed. More than a few have a sense of honor, while others believe that Cybertron would be better protected by aggressive expansion. Ultimately, the Decepticons desire to protect their homeworld, even if it is at the expense of others.