What kind of disorder is a phobia?

It is an anxiety disorder which is a continuous feeling of fear and anxiety which is disabling and can impose on daily functioning. They can be triggered by something that may seem trivial to others (possibly even non-existent things that nevertheless seem very real to the person). E.g. panic disorders, OCD, phobias, PTSD. 

Phobias essentially have a definite, persistent fear of a particular object or situation. A stimulus such as a snake, dog, or a bus stop will provoke an immediate response, which may be similar to a panic attack. Physiological symptoms include sweating, increased heart rate, shortness of breath, tense muscles or palpitations. Behaviourally, it is a fright or flight situation: you either freeze or run away. Despite the irrational sense of fear, the person still cannot control the immense terror the stimulus produces. However, this response alone is not enough to be actually diagnosed with a phobic disorder, it is only if the fear disrupts the individuals day to day life that it can be diagnosed.