The Address Resolution Protocol (ARP) is used in switches to convert an IP address to a MAC address. It is a protocol that requires a host to send a request for information, and then the switch does a lookup and gives a reply. This protocol helps devices communicate on the network. It was a protocol that was designed clear back in 1982, so it has been part of the IP protocol suite for some time. While it was not defined in the OSI framework, it is now listed as residing between layer 2 and layer 3 of the OSI model.

For more information check out this listing. For even more information read the RFC's listed as references on the Wikipedia page.

Sources: Wikipedia

This chart shows how the header is structured for this kind of ARP packet.