AUTOBOTS
The Autobots are one of the primary factions in the Transformers mythos. They usually find themselves defending both their own race and other species against the Decepticons and upholding justice and freedom throughout the galaxy. They are considered "the good guys" in most, but not in all Transformers stories.
A generally heroic, honorable group, they tend to be dedicated to peace, and feel a need to protect other races from the Decepticons. As a result, they are usually Transformers who change into non-combat oriented alternate modes such as domestic vehicles, cars, trucks, or rescue vehicles. This is not always the case, however, as their ranks certainly contain ne'er-do-wells, malcontents, and unsavory types, and the necessities of war have led many to adopt more combat-oriented alternate forms. While the Autobots have usually found themselves outnumbered by the Decepticons, the Autobots have always had home-field advantage, having not only the humans' military support, but also having more places on Earth to fall back on, while the Decepticons are entirely unwelcome on Earth.
Key concepts that repeat throughout most continuities are the use of the name Prime (or a variation thereof) to denote Autobot leadership, and the existence of an artifact of great life-giving power, usually referred to as a "Matrix", traditionally held by their leader.
DECEPTICONS
The Decepticons are one of the primary factions in the Transformers mythos. They are a malevolent race of robot warriors, brutal and merciless. The Decepticons are led by a single undeviating goal: total domination of the universe. They are typically concerned with such things as conquering Cybertron, 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, the Decepticons are led by the most powerful of their ranks. This tends to cause some conflict, given how generally every Decepticon 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.
