It's hard to keep up with real estate terminology when they're constantly coming up with "cuter" ways of calling a house a house. Sometimes referred to as a Duplex, and most recently a "Duet," is a single piece of real estate but serves to house 2 "families" or sides (by "families" I mean anyone living in the side - whether it's a single individual or a family of 5). There are several styles of Duplexes, and how they're bought/sold are can vary from state to state.

For example, in the state of New York, the most common style of Duplex (in Upstate) is a house with 2 sides (usually side "A" and "B"), which each have their own doors, but share a driveway and garage. See the picture below. You buy this as 1 property.

In Virginia, we have a different style of Duplex. We also have a very different way of buying and selling them! In VA (especially Arlington), the most common are brick colonials like the one below. However, each side has their own address (2721 and 2723), and is bought and sold individually! They usually share nothing. The most common styles have 2-3 bedrooms and 1-2 bathrooms, a wall in the backyard to separate them, their own front yards, etc. Sometime their front doors are on the sides, and sometimes they have a common front step like the one below:

(*Note: I'm not licensed in NY, but have bought and sold real estate personally there. If you're looking for a great New York Realtor, ask me!)