What is the difference between a fear and a phobia?

This is actually a good question because people confuse fear and phobia often.

A fear is being afraid of something, like climbing a mountain because you're afraid you might fall. But a phobia is an anxiety disorder and has to be diagnosed. Phobias don't pose a threat. Phobia would be a fear of the number 13 and getting anxiety when you see a 13 (it's a real phobia, too).