Let's say a guy owns a purchased video game unit (such as an Xbox or playstation, or even a handheld). He purches a "mod chip" or mod device that he physically attaches to his game unit. After it's been attached, the unit has various different features the most popular of which are: (1) play COPIES of games: Either a person rents a game, makes a dupe on their home computer, and then plays the duped game, or the person downloads copies of games off the internet (2) play other games: After a game unit has been mod'ed you probably can put other games on it, possibly games that weren't written for that unit. For example, a person might be able to put MAME on their xbox and play all the old arcade games from the 80's on the xbox. (3) play video clips, movies, pictures, etc.: Though some modern devices, (like the PS3) now have this feature built-in, not all devices do. So, for example, a mod'ed Gameboy might be able to play movies and MP3's. (4) play import games: An American xbox may not be able to play a Japanese game unless it's mod'ed. (5) Enter cheat codes: sometimes mod chips allow you to directly enter cheat codes into games. (6) enhanced system performance: Sometimes mod chips claim to be able to make a device run faster/better/more reliable.
There are no real mod chips for the 360, you basically flash the rom on the drive allowing it to read ripped and burned games. Xecuter has said that they will not make a modchip for the 360 due to legal reasons. I do believe that some lesser companies have put together a modchip for the 360 but it doesn't have near the functionality that the original xbox modchips had.
I might be wrong or "old school" about that one. I think there were mod chips with cheat codes for the original Playstation and stuff, but that might have been long ago...
the iextreme firmware was created specifically so you wont get banned from xbl. ive had it for i cant remember how long, play online every day, no problems