So tell me what is a AirPort card and do i need one and whats does it do...HELP

Its a card for wireless networking

If you have an earlier iMac (g4 or g5) chances are you do not have a wireless card which Apple refers to them as AirPort cards. If you already have a wired network - you may not need it.

A AirPort card is a wireless card for Mac's and all the new models have them built in so if your buying a new Mac its already inside it. And it allows you to connect to any wireless internet network allowing you to surf the internet from any hotspot provided you know the wep password (if it is a password protected newtork).