Wired Equivalent Privacy (WEP) is a security protocol used in wireless networks and encrypts all transmissions. It can be used in three different levels of encryption: Off, 64-bit (same as 40-bit), or 128-bit.
However, WEP has several important weaknessess which makes it very easy to crack. For example, it is vulnerable against collision-based attacks, and the number of possible combinations is only 16.7 millon.
Still, WEP has been the major encryption technique in wireless networks for more than ten years.
Wi-Fi Protected Access, abbreviated WPA was designed to be a improved version of WEP, where the security flaws have been sorted out.
For example, WPA features improved data encryption through the temporal key integirty protocol, TKIP. Also, WPA supports a 48 bit initialization vector (IV), which means 500 trillion possible key combinations.