They are the parts of the building that are permanently in place and which cannot be removed without damage to the building. The plumbing system is an example. Usually, fixtures are included when you buy a building.
On the other hand, items that are not attached or are easily removed are considered to be personal property. Office equipment is an example. Usually, personal property is not included when you buy a building, unless specifically listed in the sales contract.
Some items (like a window air conditioner) might fall into either category. To avoid disputes, it is a good idea to specify in the sales contract how such items are to be dealt with.