The Electronic Entertainment Expo, commonly known as E3, is an annual trade show for the computer and video games industry presented by the Entertainment Software Association. It is used by many video game developers to show off their upcoming games and game-related hardware. The new E3 Expo is run by International Data Group instead of by Entertainment Software Association commonly known as ESA.

E3 is largely regarded as the world's largest regular convention for video games. Video game companies generally spend more on their presentations for E3 than any other convention. This includes fancy decorations and pyrotechnics. E3 is is the same expo where Nintendo declared Wii and Sony showed playstation to public.

E3 2009:

The Electronic Entertainment Expo 2009, commonly known as E3 2009, is the 15th Electronic Entertainment Expo held. E3 2009 is scheduled for June 2-4 at the Los Angeles Convention Center. E3 is an annual trade show for the computer and video games industry presented by the Entertainment Software Association (ESA). It is used by many video game developers to show off their upcoming games and game-related hardware.

For more info on E3 2009, please look at the video:

References:
1. Wikipedia
2. IGN
3. Gamespot