A duplex apartment is a type of multi-family home that has two separate living units within one structure. The two units share a common wall, but the floor plan can vary. Duplexes can be situated side by side or stacked one on top of the other. There are separate entrances for each unit, and sometimes there are separate garages and yards as well. The interior layouts of the two units are usually identical, and they always have a shared wall. Duplexes are generally more like detached houses that are split in two, with only one wall usually shared between the two units. In contrast, a condo might resemble a regular apartment in a multi-unit building, with several shared walls, floor, and ceiling.

In some big cities, where apartment buildings are the norm, "duplex" takes on a slightly different meaning. It often refers to an apartment that spans two stories. An interior staircase or sometimes a private elevator connects the floors, and each floor often has its own entrance into the buildings hallways. So in this case, the "duplex" refers to the number of levels of a single-family unit, not to a number of units.

Overall, a duplex is much more like a detached house thats split in two. Only one wall is usually shared between the two units. In contrast, a condo might resemble a regular apartment in a multi-unit building, with several shared walls, floor, and ceiling. Condo holders only own the interior of their unit, too. The rest of the building is owned by a homeowners association.