WPA2, or IEEE 802.11i-2004, was an ammendment to the original 802.11i standard to address security problems with Wired Equivalent Privacy (WEP). In 2001, attacks that could break the WEP protection were reported and very quickly the attacks evolved to a state where a network can be compromised within minutes. Wi-Fi Protected Access (WPA) was introduced, using the Temporal Key Integrity Protocol (TKIP) as an upgrade to preserve security and privacy. WPA was meant as a solution for maintain security up until WPA2 was introduced.
WPA2 brought the the Advanced Encryption Standard (AES) block cipher to the standard, greatly increasing the security strength of wireless networks using the technology. As of 2008, WPA2 does not have any major security concerns associated with it. WPA's use of TKIP however has revealed some weaknesses. Martin Beck and Erik Tews, two graduate students in Germany, reported some weaknesses with TKIP that could be used to compromise some communications, but not to compromise the network's security fully.
WPA2's use of AES shields it from such vulnerabilities for the time being.
More information: Wi-Fi Protected Access (WPA)