From LQWiki

Wired Equivalent Privacy (WEP) is a protocol for encrypting computer transmissions via a wireless link. It does this by encrypting each TCP/IP packet before being sent and then decrypting it once it reaches the other end of the link. Due to the nature of wireless networks each individual packet has to be encrypted because not all packets will travel through the link at the same time. However, a flaw in the way encryption was implemented results in packets which occasionally re-use a portion of the same encryption key, called the initialization vector or IV. Multicasting leads to packets being sent along different channels and packets may get lost or corrupted and need to be resent. It is this design that leads to the underlying fault of WEP.

If a malicious cracker wants to gain access to your wireless network they can sit within range of it and start capturing packets. It would take anywhere from half an hour up to several days (depending on traffic) to gather enough packets to attempt the cracking of the encryption. Depending on the strength (either 128bit or 64bit), and the speed of the machine, the cracking attempt can take anywhere from an hour to a day or two. With more and more powerful machines, this time decreases.

How can one reduce the risk of having their wireless network cracked? Well first off, don't use an easy password. Numbers and letters are better. The first passwords that will be tried are plain words. Next, make sure you use at least the 128bit encryption. If your cards and software support it, try and use WPA. Another way to protect yourself is to use VPN but its not that easy to set up and therefore might not be suitable for the average home user.