What are squatters and what do they do?

Answer 1

Squatter is an informal term for a trespasser.

A squatter is a person who lives upon premises owned by another person, without the owner's permission, i.e., a trespasser. Squatters take up residence on property that belongs to someone else and treat the property as their own. In some jurisdictions, they can eventually acquire legal title by adverse possession as long as they meet certain legal requirements.

In former times squatters were people who lived on the common land. They didn't own any land and could be forced out of where they were living at any time.