What is ARP in Unix and ARP cache?

The ARP(Address Resolution Protocol) Maps Internet Protocol(IP) maps Addresses to MAC(Media Access Control) address. Basically if a sending machine knows the IP Address of the receiving machine it asks the receiving machine on a local area network to send the MAC Address of the receiving machine to the sending machine. The sending machine caches or "saves" this IP to MAC mapping and then sends information over the local area network to the receiving machine.