What is dhcp relay agent? how to configure in windows server 2008 and linux OS?

The DHCP Relay Agent component relays DHCP messages between DHCP clients and DHCP servers on different IP networks. Because DHCP is a broadcast-based protocol, by default its packets do not pass through routers. A DHCP relay agent receives any DHCP broadcasts on the subnet and forwards them to the specified IP address on a different subnet. The DHCP Relay Agent is compliant with RFC 1542, "Clarifications and Extensions for the Bootstrap Protocol." For each IP network segment that contains DHCP clients, either a DHCP server or a computer acting as a DHCP Relay Agent is required.

reference:http://technet.microsoft.com/en-us/library/dd469685.aspx