CARP is an acronym for Common Address Redundancy Protocol. It is a free alternative to the Virtual Router Redundancy Protocol (VRRP) and the Hot Standby Router Protocol (HSRP).
The primary purpose of CARP is to allow multiple hosts on the same network segment to share an IP address. CARP works by allowing a group of hosts on the same network segment to share an IP address. This group of hosts is referred to as a redundancy group. The redundancy group is assigned an IP address that is shared among the group members. Within the group, one host is designated the master and the rest as back-ups. The master host is the one that currently holds the shared IP. It responds to any traffic or ARP requests directed towards it. Each host may belong to more than one redundancy group at a time.
[Adapted from OpenBSD.org]