An apartment is a group of housing units in one building all owned by one entity. In other words, all the units are owned by one entity. The units are then rented out to tenants. This is the key difference between a condo and an apartment. With a condo, individual entities (i.e. person or corporation) own the units, whereas with apartments, all the units in the building are owned by one entity.
(pictured below is the Kennedy House located on 1901 JFK Boulevard in Philadelphia. Jeff Snader used to work there during his high school years renovating kitchens & bathrooms, painting, and also some minor electrical and plumbing work.)