Manipulating Unix Background Processes
The HUP (hangup) signal is issued to the process once the user's connection to the server is terminated. Once the HUP signal is issued, the process is terminated along with the connection, which presents one with an interesting problem; how does one keep a process running in the background?
There are a few options you can choose if you're ever presented with this problem, one is to leave the SSH session open at all times and hope that it's neither terminated, nor the computer on which the SSH session is running shuts down. Hilarity aside, I think it should be stated that this isn't a feasible solution to the problem at hand. Sure, you can leave the session open and pray there's no power outage in your general area, but don't brag about this strategy to your peers anytime soon lest you be ridiculed for blatant stupidity and/or noobishness.
Another way to get around this problem, which Doug pointed out, is to use nohup, a Unix command that is used to run another command while suppressing the action of the HUP signal.