Replay attacks, also known as repeating attacks, repeat a legitimate transmission in a malicious context. For example, a user might send their authentication information to a client or system; the attacker who eavesdrops on this communication can use the authentication in a later transmission, essentially impersonating the victim. In wireless networking, replaying transmissions can be used to enable several different attacks, including the WEP cracking process mentioned previously. In these attacks, the attacker can generate many ARP requests using a client's spoofed MAC address in order to obtain a repeated IV.
Note: Do not confuse a replay attack with a relay attack. In a replay attack, a legitimate network packet or frame is retransmitted repeatedly. In a relay attack, an attacker inserts themselves man-in-the-middle style between two devices, intercepting and forwarding traffic between them.