Desktop Tower Defense is an example of a Tower Defense game. It is pretty amusing and quite simple in concept, but takes some skill to get right. These games rely on old school fun rather than flashy graphics.
The idea is to funnel waves of enemies that are trying to get from one side of a map to another. You have to set up your defenses to funnel them through and make the most of your spenditure. As you do away with more badies, you get more spenditure to increase your defenese or upgrade them.
A website with a bunch of tower defense games is Tower Defense. Have fun.