Moles occur when skin cells grow in clusters. Those skin cells are called melanocytes, which also make the pigment of our skin. Moles are usually black or brown, and can grow anywhere during the first three decades of life.
Moles can be categorized by presentation in terms of time of onset, depths of involvement, typical or atypical presentation. Most importantly, some change, some do not, and some might disappear over time. It is important to note that if you notice irregular changes to a mole, you should see a specialist and have it diagnosed.