Before it was gamescom it was the Games Convention, a Leipzig, Germany-based show started in 2002 by the Bundesverband Interaktive Unterhaltungssoftware (Federal Association of Interactive Entertainment Software). Leipzig, though, was too small for the massive convention, so it changed its name, dyed its hair, and hit the road in 2009. The newly minted "gamescom" moved to the much more accessible Cologne, Germany, and continued to expand its yearly convention.
The show is still actually fairly new as far as gaming conventions go. Since moving to Cologne the trade show has exploded in popularity - in fact, because it's open to the public (unlike E3), it's technically the world's largest gaming show, with nearly 300,000 visitors and 550+ exhibitors filling the Cologne Trade Fair's halls with awesome gaming goodness.
Thought all of the big gaming reveals came from E3? Wrong. Each year, gamescom has led to massive gaming announcements, from new hardware to new games and trailers. In 2009, Sony revealed the PS3 Slim and PSP minis, as well as details on the European video store for the PlayStation Network. Microsoft, too, took the show as an excuse to reveal Fable III, and the XBLA versions of Fable II (which were broken down into episodic sections for digital distribution). Future years continued to impress, with Insomniac Games and other developers announcing big titles in August.
This year there are rumors that we'll hear about a new PS3 model (said to be an "ultra-slim" model) as well as speculation about different reveals from all of the big three console makers. Keep an eye on GamesRadar for more news as the show kicks off from August 15-29.