Second guy is definitely wrong, it comes built in! My 3 year old iMac came with it built in.... and specs can be found here:
http://www.apple.com/imac/features/
The new iMac has the same and more features than the old one. I have a 2 year old iMac. It came with built in wireless type "n". I use a bluetooth keyboard & mouse. The only thing plugged into the iMac is my power cord. There's a minimal desktop footprint and no cables to get tangles.