Wired Equivalent Privacy (WEP), part of the IEEE 802.11 standard, is a system to secure WiFi networks.
It comes in three main levels of security: 64 bit, 128 bit, and 256 bit encryption. Each level is more secure than the previous -- 128 bit encryption is much stronger than 64 bit.
WEP provides an adequate level of security for most home networks, but it is by no means completely secure. A WEP network can be compromised simply by passively gathering and analyzing packets as they are transmitted through the air. This type of analysis can be performed by a standard personal computer with off the shelf hardware and freely available software. A 64bit WEP network can be hacked in minutes if the traffic is reasonably high. A 128 bit network will take a few hours to crack. 256 bit networks take much longer.