Most moles occur when a group of cells called melanocytes grow together in a cluster instead of being spread out across the skin, according to WebMD. Melanocytes are the cells responsible for skin pigment, which is why they are darker than the surrounding skin.
Most moles are not dangerous and appear during the early parts of a person's life, before the age of 30, according to WebMD. Some moles change over time, while others remain the same. Still others can disappear as a person ages. If a mole appears different from others on a person's body, then WebMD recommends getting it examined by a doctor. Moles with unusual colors or shapes have a greater chance of developing into a melanoma, which is a type of skin cancer.