Melanoma is a type of skin cancer. It is the most serious type of skin cancer because melanomas have a tendency to spread quickly to other parts of the body (metastasize). Melanoma begins when melanocytes (pigment cells) in the skin become more abnormal and start to divide without control or order. These abnormal cells can invade and destroy the normal cells around them. The abnormal cells form a growth of malignant tissue (a cancerous tumor) on the surface of the skin.