Imagine there's a man in your brain who's job is to switch your train of thought from one subject to another. When you're at work he switches your thoughts to work, if you're in danger he allows you to think of how to get out, you if think of something nasty he can push it aside, setc.
Now imagine that man keeps falling asleep on the job. Worse, sometimes he switches to a negative thought first and then falls asleep.
Now you can't control your own thoughts. You linger on a negative "what if" scenario for hours, whether you like it or not. It's intrusive, and incredibly overwhelming. Sometimes it makes you obsess over tiny things, like Wilson with his laces, as if it's become a compulsion. Hence, obsessive-compulsive.
It can manifest in different ways. Sometimes it stems from a fear, like a fear of fire causing a sufferer to obsessively think about the agony of burning to death for days. Or it might manifest in silly habits, like cleaning an airport terminal. The man in your brain puts that thought up, and doesn't remove it.
The type of thoughts, the frequency, the degree of compulsion, the way it manifests, are all different for different sufferers at different times.
