WEP, or Wired Equivalent Privacy, is a wireLESS standard for protecting data transmited over a WLAN network. Since wireless signals run over the air, they may be tapped easily. These data may include your login credentials to websites or application, sensitve emails, etc.
WEP uses a key which the user must enter into the router as well as all participating nodes. The key is then used to allow the user on the network and subsequently encrypt all trafiic using the key. Users will still be able to descrypt and see the data sent by another user on the same network, just as if the user had physical access to the Ethernet wire on the wired version. The problem with WEP is that it is not secure; by intercepting a big number of encrypted packets a cracker is able to crack the key used. There are also other known problems with WEP that cannot be solved with a bigger key.
After discovering this major security problem, WPA (Wi-FI Protected Access) was quickly created to replace WEP. As the 802.11i specifications was complete, WPA2 was introduced to comply with the new standard. WPA allows for two modes of operation, a “Personal” mode, where a Pre-Shared Key (PSK) is used for authentication and encryption, or an “Enterprise” mode where a IEEE 802.11X authentication server is used. 
The personal mode works similar to WEP, all users enter the “Network Key” to gain access to the network, then all traffic is protected. This scheme is suitable for home networks and small offices, where there are few machines and seldom changing.
No chance to try the authentication server mode yet…