In a nutshell, a WPA/WPA2 network will any network card that supports WPA or WPA2 to connect to it whereas a WPA2 only network locks out network cards that only support the newer standard. WPA is both forward and backward-compatible and is designed to run on existing Wi-Fi devices as a software download. The Wikipedia article cites a white paper from the Wi-Fi Alliance:

This advanced protocol will not work with some older network cards. The later WPA2 certification mark indicates compliance with the full IEEE 802.11i standard. Some network cards can’t connect to WPA2 networks. I believe that WPA was the original WPA standard, and WPA2 was an improved version of it.