WPA = "Wi-Fi Protected Access". A good short outline of what it is:
Most of the publically-available documentation can be found starting at:
In brief, WPA changes WEP to use:
per-session keys a different initialization vector scheme per-packet key changes (based upon the per-session key) a message integrity code (TKIP)
WPA is the precursor to full-on 802.11i security. That standard, which the marking folks will call WPA2, is due out later this year. 802.11i/WPA2 will -- among other things -- will probably require AES encryption instead of RC4 for packet encryption (which older AP hardware usually will not support, thus upgrading to WPA2 will often mean replacing older APs with newer ones).
WPA started appearing on new access points (and as firmware upgrades to some existing APs, including many Cisco models). Cisco already had its own proprietary scheme, LEAP. But now most Cisco APs can alternatively do WPA.