Address Resolution Protocol is used for discovering Data Link layer addresses, like MAC addresses from IP addresses. It is used whenever a device wants to communicate with another device in the same LAN.

ARP cache is created when an IP address is resolved. It is stored on any system in IPv4 Ethernet network. Entries can be made manually.

Proxy ARP is a technique by which a proxy device on a given network answers the ARP request for an IP address that is not on that network. The proxy is aware of the location of the traffic's destination and offers its own MAC address as the destination.

Gratuitous ARP is almost like an administrative procedure, carried out as a way for a host on a network to simply announce or update its IP-to-MAC address. Gratuitous ARP is not prompted by an ARP request to translate an IP address to a MAC address.

Host machines that do not know their own IP address can use the reverse address resolution protocol (RARP) for discovery.

Whereas ARP uses an IP address to find a MAC address, inverse ARP uses a MAC address to find an IP address.