Originally Posted by ReverendTed
The Wikipedia links should explain everything, but the short answer is:
An Easter Egg is a "hidden" bit of content in a given item of (typically interactive) media.
For instance, a game might have a hidden room with a special message from the developers, or a DVD might have a hidden menu item that plays a special video clip.
or sometimes they're references to other viddeo games(for instance in Reality 2.0 Sam will sometime in the game yell" It'sa me Sam" jump up high in the air and get 3 Mario coins)