The major differences between apartments, condos and townhouses involve how much of the property is owned by the tenant. HowStuffWorks describes the differences relating to the ownership of airspace, structure and the land underneath the property.
According to HowStuffWorks, a townhouse is a residence that is built to share structural elements with other townhouses. Ownership of a townhouse entails control over the parts of the structure unique to the individual unit and the land underneath it. Control of the townhouse is typically split between its owner and the neighborhood association that controls multiple surrounding properties.
Ownership of a condo is different in that the owner of the unit is responsible only for the airspace between the interior walls. Typically, all exterior elements of the condo development are held in common by the condo board association and decisions relating to land use, landscaping and renovations must be cleared with the board.
An apartment is entirely owned by the landlord or rental management company. Tenants rent apartments and have no claim to ownership of the structure or the land it is built on, according to About.com. Apartments are distinguished from other rental properties by being fully integrated with a larger residential structure.