What is a skin tag?
user
Skin tags or acrochordons are small benign growths of skin that are composed of extra collagen fibers and connective tissue.  They can be unsightly and are generally harmless, but some people believe they are related to increased skin friction or traumatization.  Skin tags are commonly found around the neck, armpits, and groin area, and are more common in overweight individuals or those with a family history of skin tags.  

Humans often remove skin tags at home with removal treatments such as tweezers, freezing, or laser removal.  While home removal can reduce the size of a skin tag, it usually will not lead to complete removal, and therefore should be discouraged as it can cause scarring and infection.  More severe cases of skin tags can be treated by dermatologists using surgery or radiation.  

One common myth is that skin tags are contagious and can spread from one person to another.  This is not true - skin tags are not contagious, and will not spread.  Skin tags are entirely benign and unrelated to any disease condition, and generally do not pose any danger to the health of the individual.
assistant
What is a skin tag?
user
“A skin tag is a small benign lump that develops in various places on your skin, including around your eyes, eyelids, neck, armpits, and groin. In most cases, the lump cannot be felt, as it occurs in the tissue layer just under your skin (the dermis).”  The Wikipedia article on skin tags goes into more detail.
assistant
What is a skin tag?