A phobia is an excessive or unreasonable fear of an object, place or situation. Simple phobias are fears of specific things such as insects, infections, flying. Agoraphobia is a fear of being in places where one feels "trapped" or unable to get help, such as in crowds, on a bus, or standing in a queue. A social phobia is a marked fear of social or performance situations.
Phobias are extremely common. Sometimes they start in childhood for no apparent reason; sometimes they emerge after a traumatic event; and sometimes the develop from an attempt to make sense of an unexpected and intense anxiety or panic (e.g. "I feel fearful, therefore I must be afraid of something").
When the phobic person actually encounters, or even anticipates being in the presence of the feared object or situation, s/he experiences immediate anxiety. The physical symptoms of anxiety may include a racing heart, shortness of breath, sweating, chest or abdominal discomfort, trembling, etc. and the emotional component involves an intense fear - of losing control, embarrassing oneself, or passing out.
Commonly people try to escape, and then to avoid the feared situation wherever possible. This may be fairly easy if the feared object is rarely encountered (e.g. fear of snakes) and avoidance will not therefore restrict the person's life very much. At other times (e.g. agoraphobia, social phobia) avoiding the feared situation limits their life severely. Escape and avoidance also make the feared object/situation more frightening.
With some phobias the person may have specific thoughts which attribute some threat to the feared situation. This is particularly true for social phobia where there is often a fear of being negatively evaluated by others, and for agoraphobia when there may be a fear of collapsing and dying with no one around to help, or of having a panic attack and making a fool of oneself in front of other people.
With some phobias there may be accompanying frightening thoughts (this plane might crash; I'm trapped; I must get out). However with other phobias it is more difficult to identify any specific thoughts which could be associated with the anxiety (e.g. it is unlikely that a spider phobic is afraid of making a fool of themselves in front of the spider). With these phobias the cause seems to be explained more as a conditioned (learned) anxiety response which has become automatic.