The term "Easter eggs" refers to features that are hidden inside of a software program, DVD, or other such technology. For example, back in the original Nintendo days, there was a game called Contra. You started with three lives, and that was a pain. However, you could also enter a code (now known as the Konami code - after the game manufacturers) to give you thirty lives. It's been years since I played the game, but I still remember you had to hit up, up, down, down, left, right, left, right, B, A, and then Start, and you would start the game with thirty lives. That was my first experience with Easter eggs. Nowadays, you can find many sites that list the Easter eggs in video games. Aside from video games, many DVDs now come with Easter eggs built in, as you can find on this site.
There are typically Easter eggs built in to software, as well. For example, Microsoft Excel 97 used to have a flight simulator built in to it, but Microsoft apparently removed Easter eggs in their software in 2002. Note that sometimes people get confused as to what is a hidden feature and what is an Easter egg.
Anyway, even Google has some Easter eggs in it, and I saw an article showing some of them.
Top 15 Google Easter eggs