Windows Server 2003 Question:

A DHCP server is the server that is responsible for assigning unique IP address to the computers on a network. No two computers (actually, no two network cards1 [even if two are in one computer]) can have the same IP address on a network at the same time or there will be conflicts. To that end, DHCP servers will take a request from a computer that has just been added (or is renewing) to the network and assign it a unique IP address that is available. These assignments typically only last for a limited time (an hour to a week usually) and so you are never guaranteed that the IP address for a particular computer will remain the same when using a DHCP (some DHCP servers allow you to specify that a computer gets the same address all the time however).