The cells that make skin pigment (melanin) are called melanocytes and these can form cancerous skin tumours known as melanomas. In reality, most people have moles which are benign (non cancerous).
By contrast a melanoma that is left is likely to prove fatal. Whilst they can occur anywhere, they can also occur on the foot and beneath the finger or toenails. Beware dark discolouration when there is no history of trauma.
There is plenty of information available on the internet and some links are included.
The best thing you can do is NOT TO IGNORE new or changing lesions, seek a professional opinion. Early recognition and excision will cure the problem.