Moles can be a growth, usually light brown, and typically on the face.
Thickening of the epidermis can produce different types of "warts", either viral warts (verruca, mollusks), solar keratosis (chronic sun-damaged skin), sebaceous warts (old man's warts) or skin tags.
Skin cancer can be black balls, moles with many colors, large moles, bleeding or ulcers in the skin.
Lumps or skin changes that change over the course of a few months can be an indication of skin cancer.
Growths can also come from deeper layers of the skin such as cysts and fat globules.
The reasons vary with the type of growth.
Read more about the different types of growths and their causes under the respective articles.
GPs can usually give an answer as to whether something is dangerous or not.
If there is any doubt, the GP can cut a sample (tissue sample) or refer to a dermatologist.
Serious skin cancer is treated surgically, while "benign" skin cancer can be treated with freezing or light. Warts can be frozen away.