This is characterized by relating larger size, irregular borders and irregular coloration:

Somewhat different in appearance, this is also a form of melanoma, characterized by a very darkly pigmented nodule.

The next three pictures are NOT melanoma, but are benign moles (nevi):

This is the type of mole called a blue nevus, a mole with blue coloration. Common blue nevi are harmless but may mimic melanoma in appearance. One type of blue nevus, called cellular blue nevus, has a malignant potential.

This is a compound nevus, the most common type of mole.

This is a congenital pigmented nevus, a nevus which is apparent at birth. Some congenital nevi have a malignant potential.