What is the difference between wanting something really badly and having a compulsion?

Function!: All of us at times want something "badly". How we get it, or how we try to get it and don't succeed but keep trying- no matter the cost in time, money and any destruction it causes to our lives-is the difference. Generally compulsions carried to extremes drive us to do the same thing again and again and expect the results to change. That's a malfunction!

It's the intensity: Of the distress. A compulsion (behavior) or obsession (thought) comes with intense anxiety & distress. Wanting something really badly or having a goal usually feels more positive and motivating & is something you think is good for you. If you get what you want, you feel good. A compulsive action is never satisfying, it just keeps repeating & the distress continues.

Somewhat alike: A compulsion is a repetitive behavior that is in response to a thought or urge. The implication is that it is powerful and very difficult to resist. Compulsions can be unwanted or intrusive and tend to feel out of control.