Key Difference: Mania is basically a type of mood which is characterized by abnormally high energy levels. Mania has a variety of intensity levels, which range form mild mania to full mania with extreme energy, racing thoughts, and forced speech. Phobia refers to an abnormal fear of something. Phobias are actually a type of anxiety disorder, which basically means that when a person comes into contact with their phobia, their anxiety levels rise.
Put in short, a mania is a crazed obsession or belief. A phobia is a strong revulsion or fear that has no basis in fact or even reality. Some people can have a phobia of snakes without ever having even held or seen one.