A birthmark is a congenital, benign irregularity on the skin which is present at birth or appears shortly after birth—usually in the first month. They can occur anywhere on the skin. Birthmarks are caused by overgrowth of blood vessels, melanocytes, smooth muscle, fat, fibroblasts, or keratinocytes. Dermatologists divide birthmarks into two types: pigmented birthmarks and vascular birthmarks. Pigmented birthmarks caused by excess skin pigment cells include: moles, café au lait spots, and Mongolian spots. Vascular birthmarks, also called red birthmarks, are caused by increased blood vessels and include macular stains (salmon patches), hemangiomas, and port-wine stains. A little over 1 in 10 babies have a vascular birthmark present by age 1. Several birthmark types are part of the group of skin lesions known as nevi or naevi, which is Latin for "birthmarks". Birthmarks occur as a result of a localized imbalance in factors controlling the development and migration of skin cells. In addition, it is known that vascular birthmarks are not hereditary. Pigmented types
Mole
Congenital melanocytic nevus is a type of  melanocytic nevus, the medical term for what is colloquially called a "mole", found in infants at birth. Occurring in about 1% of infants in the United States, it is located in the area of the head and neck 15% of the time, but may occur anywhere on the body. It may appear as light brown in fair-skinned people, to almost black in people with darker skin.
Children are also said to be marked by some sudden fright or unpleasant experience of the mother, and I have myself seen a pop-eyed, big-mouthed idiot whose condition is ascribed to the fact that his mother stepped on a toad several months before his birth. In another case, a large red mark on a baby’s cheek was caused by the mother seeing a man shot down at her side, when the discharge of the gun threw some of the blood and brains into her face.Other explanations claimed that birthmarks shaped like food were the direct result of the mothers pregnancy cravings, or the mother touching a certain part of her body during a solar eclipse - her childs birthmark will be in the same location. References
External links

Birthmarks via the Cleveland Clinic
Vascular Birthmarks Foundation
Birthmarks via MedlinePlus