A faction of Cybertronians who waged war against the Autobots for control over the Allspark, the control of their world's supply of Energon and utimately so they could conquer the universe.
The first true Decepticon was the Fallen, a renegade Prime, responsible for the fall of the Original Thirteen Primes. Despite the damage he caused he was sealed away and cast into another dimension in a sarcophagus prison. An untold time later, when Sentinel had reestablished Cybertronian civilization, the Fallen's prison was found. Sealed away he worked to poison the mind of Lord High Protector Megatron, turning him against his comrades and into his slave, forming the Decepticons.
They then went on to wage war against the Autobots for the Allspark. Eventually leading them to Earth, after the battle of Egypt and Chicago, the Decepticon have been devastated.
However, several Decepticons later found themselves revived in another Universe, where their species does not naturally exist.