The Decepticons were Megatron's followers and were mostly gladiators. Now they the evil enemies of the Autobots. Their home planet is Cybertron where wars between the Autobots and Decepticons has depleted it of it Energon, scared the planet, and leaves it almost uninhabitable. They traveled to Earth in search of Energon the revive their planet.
In in season one and two, their base is the Nemesis, a space warship. In two's season finale, the Nemesis fires and destroys the Autobots' base and they also created Megatron's "New Kaon", Darkmount. They used Darkmount until it was destroyed by the Autobots.