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Address Resolution Protocol. It's part of Ethernet and translates a 48 bit MAC address to a 32 bit IP address.

ARP who-has

In tcpdump one may have seen arp messages that say "arp who-has tell" or similar. This is an ARP request. It is usually broadcasted to MAC address FF:FF:FF:FF:FF:FF on the local LAN.

ARP is-at

Like above the answer or ARP reply to the above request is "arp reply is-at 00:11:22:33:44:55". This message is usually unicasted.

Self ARP'ing

Some network devices ARP themselves such as "arp who-has tell" and this is also broadcasted. This usually occurs when an interface is turned up with a certain IP. If they get a reply they will log this locally somewhere.

ARP Poisoning

ARP poisoning is an attack that rewrites the destination MAC address of another host and "poisons" the ARP cache of the attacked host.

Looking from the outside in

If someone is looking at frames from a Wifi connection they will only see encrypted frames. However a weakness of all Wifi protocols is that all frames are not the same size. This lets one identify with some certainty what an ARP packet is by judging a frames size. This was to the detriment of the WEP encryption method on wireless links because these frames could be replayed. Replaying was important for gathering IV's.