WEP stands for Wired Equivalent Privacy and was the first encryption used in Wifi (WLAN). It has been replaced with WPA2, however most wifi cards still support WEP for whatever reason.
The plaintext payload has a 4 byte (32 bit) CRC trailer. The message is then XOR'ed against a keystream. The ciphertext is then prepended by a 24 bit IV.
24 bit IV
A random 24 bit IV is prepended to every encrypted payload (every frame). Given its limited size there will be IV collisions after so many frames. A collision means that the value is the same for two or more IV's. When an IV is the same that means that the keystream between those collision packets is the same as well.
Fluhrer, Mantin, and Shamir attack
This is a successful attack (also known as FMS attack) against WEP. It is used by the program aircrack-ng. Due to this WPA was invented supposedly.
Replay attacks are possible with WEP, and they are the reason that people can replay ARP requests and gather IV's (and IV collisions) that way.
Because the payload has a 32 bit CRC at the end someone can make a replay with an IP address changed. One needs to recompute the IP checksum which is 16 bits and recompute the 32 bit CRC at the end of the plaintext payload (802.11 header + IP packet + CRC32 checksum). Most home routers should allow outgoing packets...