A house that supports more than one unit is considered a multi-family house. These houses may be designed this way, as a duplex, triplex, or townhome, or they may be a retrofitted single-family home. These houses are sometimes sold as units, like condominimums, to different owners, but are more often sold to an individual owner who will either live in one unit and rent the other portions, or use the entire property as an investment.
When looking for multi-family homes, investors will often look for a place that will pay for its own expenses. The best case scenario means that one unit can be vacant and still all the bills for the place are paid. This makes it more cost-effective to own a larger property with several rental units inside.
From a living standpoint, multi-family homes have some benefits and drawbacks. Utilities like heat are often lower for each unit, as heat may transfer through shared walls. These same walls may also transfer noise, however, which doesn't always work well when different tenants have very different hours.
If you're buying an older home that has been subdivided, there are some things to consider when you purchase the place. Many of these places are from the turn of the century, which means looking at things like asbestos and lead paint, as well as a sturdy foundation and functional roof. The good thing about an older home is that there are often hand-crafted artistic components that are unique, and sturdy hardwood construction throughout the home.