Wisdom teeth are what the third molars are normally called, appearing around age 17-25. Most adults have 4 wisdom teeth, one in each corner of the upper and lower jaw.
It is possible for a person to have more than 4 wisdom teeth, and these are called supernumerary teeth. As well it is possible to have no wisdom teeth.
There are many reasons that wisdom teeth need to be extracted, such as impaction, resorbing other teeth, or becoming partially erupted. When there is not enough space for the third molar to fully erupt or when they are abnormally positioned, it can lead to infection and inflammation of the surrounding gingival tissue — Pericoronitis. Therefore, removal of the third molars would be suggested. In most case dentists try to prevent this unnecessary infection and pain, by treatment planning earlier extraction of the third molars.