What Wireless Network Adapter Do I Get??

I am a complete n00b to wireless networking. Which is scary because I now manage one! (Don’t tell anyone at the office my secret!) Anyways, I need to buy a new wireless network adapter for my bosses laptop and I am a bit confused. Whats the difference in the letters below? And how can I find out what one to choose?

802.11a, 802.11b, 802.11g, 802.11n