An obsession is a thought or idea that keeps coming into your mind even when you don't want it to. An obsession is annoying, unpleasant, you feel silly about it, you wish it wasn't there. An example of an obsession is the idea that your hands are dirty and that you might become ill even though you really know deep down that they are not.

Compulsions are things you feel you have to do even though you really don't want to and you might even try to stop from doing them but this might not be possible. Often, a compulsion means doing something time and time again. For example checking 20 times that you've locked the back door, even though you know it's really locked.