ARP is the Address Resolution Protocol.
ARP and RARP
The ARP protocol is used to map IP addresses to MAC addresses.
RARP, the Reverse ARP Protocol, is used to map MAC addresses to IP addresses.
The ARP Cache
To reduce the number of ARP requests, every system which implements the ARP protocol keeps a cache of recent mappings.
Additional Reading on ARP
To learn more about the ARP cache, read What is the ARP cache?
To learn how to clear the ARP cache, read How do I clear the ARP cache?
The ARP protocol is defined in RFC 826: An Ethernet Address Resolution Protocol.
The RARP protocol is defined in RFC 903: A Reverse Address Resolution Protocol.