An Acronym for Address Resolution Protocol.
A protocol that enables a IP conversation to occur over an Ethernet network. IP (version 4) uses 32-bit addresses, usually displayed in a dotted quad format, nnn.nnn.nnn.nnn. Ethernet uses 48-bit addresses, known as MAC addresses, usually displayed as a 17-character colon-delimited hex string, nn:nn:nn:nn:nn:nn. (A real example: my workstation has the IP address 192.168.1.46 and a MAC address of 00:50:FC:B1:21:C7)
In order to send an IP packet over Ethernet, your computer needs to associate a MAC address with the requested IP address. It does this by sending an Ethernet Broadcast packet, using the ARP protocol. This packet basically says "What MAC address is associated with this IP address?", sometimes referred to as a "who-has?" request.
Although any workstation on the local Ethernet is capable of sending a response, usually it is only the actual workstation you wish to communicate with that bothers to reply, providing both its IP address and its MAC address.