Ownership" is a bundle of rights and obligations. These include the right to sell the ownership, or to divide the ownership or to prevent others from intruding upon the ownership, including possession.
"Possession" is a physical control of some property that may or may not be owned by the person in possession at that moment. Sometimes possession can mature into ownership by the intentional abandonment of the ownership by the previous owner. However, possession by anyone other than the owner can typically be terminated at the discretion of the owner.
For example, when you rent an apartment you have possession of the space within the rental and various related rights to come and go, park in the lot, etc. If the owner of the apartment wants to reclaim possession from you, there are legal procedures (eviction) for doing so. If the owner wants to prevent a criminal from entering your apartment, they may also have the right (if not obligation) to protect your possession, by using the laws of trespass against anyone not authorized to be in possession of your apartment or anything inside your apartment.
Some landlords are unclear on this concept and falsely believe they can still enter a rented apartment because "the own it", when the law would actually require them to stay out during the period in which they have granted the exclusive license of possession to you (i.e., a rental contract).