There are many kinds of phobias. Some common ones are fear of social situations, fear of flying, fear of heights, and fear of snakes. People can develop an unreasonable fear of almost anything. People have reported fear of the number thirteen, fear of peanut butter sticking to the roof of the mouth, and many other fears.
Most fears have some basis in reality.
For instance, if you almost drowned once, you may develop a phobia about water. If your father was afraid of enclosed spaces, you may have learned that fear from him. A fear is not considered a phobia until it causes you distress or it causes problems in your life. If you are afraid of tidal waves but you spend your whole life in Kansas, it will probably not be a real problem. If you are afraid of heights and you get a job on the top floor of a high-rise building, it will be a problem.