Decepticon
The Decepticons (Destrons usually, in Japanese shows and literature) are one of the primary factions in the Transformers mythos. They are typically concerned with such things as amassing large quantities of energon, developing powerful weaponry, and galactic conquest.
They are "the bad guys" in most Transformers stories.
Generation 1 and Unicron Trilogy
In the various continuities descending from Generation 1 as well as in the Unicron Trilogy, the Decepticons and the Autobots are the two great powers in the Transformer population. The factions, however, are not a complete set -- there are Transformers who are unaffiliated / neutral, or even members of smaller or splinter factions.
Many G1 timelines actually credit Megatron as the founder of the Decepticons, not merely its most recent leader. In contrast, Optimus Prime is almost always depicted as only the latest in a long line of Primes or Matrix Bearers.
The history of the Decepticons is not well-established in the Unicron Trilogy.
RiD Decepticons
In the Robots in Disguise cartoon, the Decepticons were a brand new faction that came into being during the events of the story. Because there were so few Decepticons (only six or seven), and because they were a subset of the Predacon antagonists, they can almost be viewed as a subgroup rather than a proper faction. The RiD Decepticons were created on Earth when Megatron's forces captured a set of stasis pods from an ancient Transformer spaceship that had been found on Earth, leading to the birth of the Commandos and Scourge. The toy version of Galvatron was also labelled as a Decepticon, although this "change" in the character's faction was not established in the cartoon series.
In the Japanese version of RiD, Car Robots, the Decepticons were known as Combatrons.
