"Squatters" are persons who occupy a place without permission. Squatting is based on "adverse possession", an idea which encourages the use of an abandoned property. "Squatter's rights" allow the use of someone else's property if there is no attempt by the owner to get rid of them. You can actually lose ownership of your property to a squatter. To protect your rights, you may need a lawyer, a title agent and a real estate agent; in C. James Vendetti, you have all three.
Squatting is trespassing, which is illegal, but Pennsylvania law on this, like so many