Those small spots that develop in different parts on the body and vary in colour are a set of pigmented cells known as moles in common language, or nevi in medical terminology. Here's a quick factsheet on them:

Moles first start to appear during childhood.
A normal adult may have as many as 10 to 40 moles.
They can form anywhere on the body.
Moles are harmless in most cases, though rare cases may become cancerous (melanoma or skin cancer).
A mole may/may not change in appearance and shape over time.

There are different types of moles, and depending on the type, a mole can be at a risk of developing into cancer. Mole types

Dysplastic/atypical nevi - These moles are irregular in shape and range from being light to dark brown, red or even pink in colour. They are also larger than an average mole. Though typically these moles are harmless, a person may be at a risk of developing melanoma under the following conditions:
- Has more than four dysplastic nevi
- Was previously detected with melanoma
- Has a relative who has melanoma
Congenital mole - A mole that is present on a person's body since birth is known as a congenital mole. It could be very small in size or quite large, and a person's risk to melanoma increases with the size of the mole. A mole larger than the size of a pencil eraser should be investigated for melanoma.
Spitz nevi - This mole is mostly pink, dome-shaped and raised in nature. The colour may vary from red or brown to black. A spitz nevus can also have an opening that bleeds. Though most of these types of moles develop by the age of 20, they may also form later in life.

How to know if a mole is cancerous?

Most moles are harmless. The areas of the body that are most exposed to sun require attention for cancerous moles. These include arms, hands, neck, ears and face. Those that could be a cause of concern are the ones that appear after the age of 30 or those that change colour, size or shape over time. A mole that suddenly becomes tender to touch, painful or itches, bleeds or oozes should be shown to a dermatologist (specialist that deals with skin disorders). Asymmetry in the mole's appearance, i.e. one part looks different from another, should also be shown to the doctor. Sources:"Moles, Freckles, and Skin Tags," WebMD.com, https://www.webmd.com/skin-problems-and-treatments/guide/moles-freckles-skin-tags"Moles," AAD.org, American Academy of Dermatology, https://www.aad.org/dermatology-a-to-z/diseases-and-treatments/m---p/moles"Moles," MedlinePlus, NLM, NIH, https://www.nlm.nih.gov/medlineplus/moles.html"Moles," MayoClinic.com, Mayo Clinic Staff, https://www.mayoclinic.org/diseases-conditions/moles/basics/definition/con-20019745