It is an interim solution that is used now until 802.11i comes out.
It still using RC4, but the Key was changed to TKIP.
TKIP basically works by generating a sequence of WEP keys based on a master key, and re-keying periodically before enough volume of info. could be captured to allow recovery of the WEP key. TKIP changes the Key every 10,000 packets, which is quick enough to combat statistical methods to analyze the cipher.
TKIP also adds into the picture the Message Integrity Code (MIC). The transmission’s CRC, and ICV (Integrity Check Value) is checked. If the packet was tampered with. WPA will stop using the current keys and re-keys.