A phobia is basically a fear. We all have fears about things such as heights and spiders but, for most of us, they do not interfere with the way we lead our lives. These fears are only called phobias when they interfere with things we would otherwise enjoy or do easily.
Phobias are extremely common. Sometimes they start in childhood for no apparent reason; sometimes they emerge after a traumatic event; and sometimes they develop from an attempt to make sense of an unexpected and intense anxiety or panic (eg "I feel fearful, therefore I must be afraid of something").
Some common phobias include a fear of blood, of vomiting, of birds, of enclosed spaces (claustrophobia) or of social or performance situations (social phobia).