when the GPS is enabled, does it drain battery? if the GPS is enabled, but the app doesn't use it, does it drain battery? (I understand that GPS drains battery because it requires a different component to be charged up.) Or the fact that the GPS is enabled only means that the App can use it?
If GPS is enabled at an app level it doesn't use more battery unless an app uses GPS. And even if an app uses GPS it can do so in a way that minimizes it's usage and battery drain. For example if Foursquare were to use GPS efficiently it would only fetch your location when you are checking in or looking up places to check in then turn off it's access to the GPS.

Now at a system level enabling GPS might drain the battery a little but that depends on what you have enabled, i.e. Find my iPhone.