WPA, or Wi-Fi Protected Access, is a Wi-Fi standard that wasdesigned to improve the security features of WEP (Wired EquivalentPrivacy). The technology is designed to work with existing Wi-Fiproducts that have been enabled with WEP (i.e., as asoftware/firmware upgrade to existing hardware).
The 2 major improvements over WEP:
Improved data encryption through the Temporal Key IntegrityProtocol (TKIP). TKIP scrambles the keys using a hashing algorithmand, by adding an integrity-checking feature, ensures that the keyshaven't been tampered with.
User authentication, which is generally missing in WEP, throughthe extensible authentication protocol (EAP). WEP regulates accessto a wireless network based on a computer's hardware-specific MACaddress, which is relatively simple to be sniffed out and stolen.EAP is built on a more secure public-key encryption system toensure that only authorized network users can access thenetwork.
* It should be noted that WPA is an interim standard that willbe replaced with the IEEE's 802.11i standard upon itscompletion.
Source - Webopedia.com