Explain Air Cards Please

My youngest son has a Mac Powerbook G4 titanium. 1 GHz PowerPc. 1 MB L3 Cache. 1 GB SDRAM.

He doesn't have internet in his apartment and I wanted to get him an air card for Christmas.

Would you tell me how air cards work. Are they called something else, because I don't see air cards in any of the holiday advertisements? Do they have to be "activated" through cell phone service, etc.?

As always, thanks for your help.

Daugust