An apartment duplex consists of the union of two words - "apartment" and "duplex." A long time ago, most communities would have had a common kitchen or bathroom area in large walled compounds. Many modern apartments will have their own bathrooms and kitchens.
These modern apartments are rooms set "apart from each other" within a larger building. There may be a common main secured or unsecured entrance. Shared laundry, game or community rooms may be available to all occupants. Water and garbage service is usually included in rent. Electricity may or may not be included in the rent.
Some apartments have patios or porches allowing the first floor occupants to enter or exit their units at their leisure. The upper floors might have a fire escape stairwell on the outside of the building. Each apartment will have an entrance to a central hallway. Apartment complexes can consist of a few or hundreds of units.
The "duplex" refers to "two" after the Latin word, "dualis." The apartment duplex consists of two apartments. This offers a little more privacy than a large apartment complex while remaining affordable. At times, the owner may occupy one apartment renting out the other unit making this a nice investment option.
The two units for the apartment duplex can be side-by-side horizontally or on-top-of each other vertically. There may be a common external entrance leading to a foyer or separate external entry ways. Some apartment duplexes are remodeled single-family homes.