Wired Equivalent Privacy
The Wired Equivalent Privacy or WEP protocol was the original encryption system used in IEEE 802.11 wireless networking. It is still quite widely deployed, but is now deprecated in favour of the newer system Wi-Fi Protected Access or WPA.
Some argue that the name itself indicates a fundamental design error. Why should a wireless security system be designed for only "wired equivalent" privacy, only as secure as ethernet? Clearly, the requirements are different. If any machine plugged into your wired network can sniff your traffic, that may or may not be problematic. If any machine that joins your wireless network can, that is certainly a problem. The counter-argument would be that, since WEP controls who can join the network, this is not a problem.
A number of very serious flaws have been found in the WEP system. "Wired Equivalent Privacy (WEP) isn't."  The system is completely broken.
WEP is often cited as an example showing that protocols designed in secret, ignoring Kerckhoffs' Principle, are often of low quality. "These attacks point to the importance of inviting public review from people with expertise in cryptographic protocol design; had this been done, the problems stated here would have surely been avoided." 
WEP generates keys for different connections by concatenating a connection-specific intialisation value with another secret value, and this creates a vulnerability. It can be broken very quickly by a related key attack .
At least one custom-built computer has been created specifically to attack WEP.