It's a whole thing. We had to learn about it during my apprenticeship because various laws apply in one case or another.

Possession is "having actual control over something", whereas ownership is "having a legitimate right to control something". "Legitimate" defined as "conforming to the law".

So a person who rents a house is in possession of the house. The person they rent it from owns it. An eviction is when the landlord wields violence or the threat of violence to dislodge the renter from the house.

Without legal enforcement, possession and ownership quickly collapse into one, such as in more primitive societies.