If a sender doesn't know the MAC address of the destination, it has to get that address before data can be sent. To obtain the unknown Layer Two address when the Layer Three address is known, the sender transmits an ARP Request. This is a Layer Two broadcast, which has a destination address of ff-ff-ff-ff-ff-ff. Since Ethernet is a broadcast media, every other device on the segment will see it. However, the only device that will answer it is the device with the matching Layer Three address. That device will send an ARP Reply, unicast back to the device that sent the original ARP Request. The sender will then have a MAC address to go with the IP address and can then transmit.

RARP obtains a device's IP address when it already knows its own MAC address.

This is a helpful link from where I pasted the definitions above:

http://searchwarp.com/swa9547.htm