Wisdom teeth are genetic "leftovers" from human ancestors. As human jaws slowly decreased in size because of the introduction of meat into human ancestor's diets the wisdom teeth were slowly "crowded" out. Although in a person with a large enough jaw the wisdom teeth will grow normally, in a person with a smaller jaw the teeth may become impacted and will need to be extracted.
Interestingly because they are no longer needed by humans to function, not all people have them.
Wisdom teeth are the third molars and the last ones in the series of teeth. If there is room for them to erupt, they come out at the age of 16 to 24 years old. They can be very useful for chewing if they are well aligned. However, for most people, they do not come out completely, staying impacted in the jaw and should be extracted.