The Forge is an illegal gladiatorial arena within the Cybertronian city-state of Kaon. It is within this arena that Megatron rose to prominence and began organizing the insurrection that became the Great War.
Dreamwave Comics continuity
The Forge is shown to be where Megatron trained the majority of the Decepticons, as well as applying extensive physical and psychological conditioning, bodily modification, and branding, all under the heavily bribed nose of Emirate Xeon. It's here that Grimlock met Starscream. It's also rumored that the repeated abuse seared a few new subroutines into Starscream's fetish processors.
IDW Comics continuity
In this continuity, the Forge is originally shown to be an illegal gladiatorial arena deep within Kaon, however once Prowl and his forces start to close in the locations of the fights is changed each time, with the Constructicon building the arena on an ad-hoc basis. They were also seen using a portable smelting pool (actually Mixmaster) to melt down an empty for the foundation. It is unclear if each of these arenas were designated "The Forge" in turn or if that appellation was unique to the original arena.