Wisdom teeth are the the largest of the Molar Teeth.

In the mouth of a human there are incisors (to "cut" food), canine (or dog teeth), pre-molars and molars (for "grinding" or "chewing").

The Wisdom teeth are the large molars at the end of each line of teeth - there is one Wisdom tooth at the end of each row - therefore there are 4 in total.

Wisdom teeth are the last permanent teeth to develop. They appear in the mouth 
between the ages of 18-24 years and may develop in the jaw but never erupt or appear in the mouth.