A wart is a hard, callous thickening of the epidermis in the form of a crude cauliflower-like bulge. A wart is caused through the human papilloma virus (HPV).

Are there different types of warts?
There are many types of warts. The most common are described  in What is a wart?

What to do if you are unsure if a thickening is a wart or not?
If unsure, always consult your doctor!