Wi-Fi Protected Access
Wi-Fi Protected Access or WPA is an encryption system used in later versions of the IEEE 802.11 standard for wireless networking. It replaces an earlier standard, Wired Equivalent Privacy or WEP, which is completely broken. There are different variants of WPA. The original WPA is now deprecated in favour of WPA2. WPA2 is required for full compliance with 820.11i.
There are also variants in the keying methods used. TKIP for Temporal Key Integrity Protocol was the original protocol in WPA, a stopgap required because WEP was broken; it can work on devices designed for WEP with only a firmware upgrade. Unfortunately, TKIP also has weaknesses  
WPA2 is the current standard. It has two modes, Personal or PSK mode using a pre-shared key and Enterprise mode which relies on an authentication server. PSK mode is vulnerable to a dictionary attack if a poor password is chosen. The Church of WiFi provide pre-computed tables to facilitate such attacks and there is at least one commercial service offering to retrieve keys and another offering software so you can do it yourself.