A phobia is an intense and irrational fear of something. If seeing a spider makes you scream and jump on top of a table, then you might suffer from a phobia.
The thing to remember about phobia is that it describes an irrational fear. If you’re afraid of a black widow spider, for example, that’s not a phobia — it’s just smart, because the spider is poisonous. If you’re afraid of all spiders, however, and the sight of a harmless daddy long legs spider has you shaking in terror, then you may have a phobia. In this case, it's a phobia because your reaction is irrational.