The term "townhouse" is actually derived from common language which existed centuries ago. It was defined as a place of residence of nobility; specifically in the United Kingdom. However, this term has changed over the years to include a specific type of residential dwelling.
Townhouses are generally found in urban environments and more often than not, they represent semi-detached dwellings in upscale areas. They are frequently found in rather well-to-do neighborhoods that boast low levels of crime. Thus, the prices to rent or to purchase a townhouse tend to be considerably higher than those of an apartment.
Another characteristic of many modern townhouses is that they are generally located adjacent to one another; that is, they will share one or more walls with nearby properties. Modern structures will also exhibit a relatively small physical footprint and contain multiple floors. Although exterior gardens are not entirely common, some townhouses may include these outdoor amenities are part of their property.
As the name may denote, these semi-detached "houses" are generally self-contained. All of the normal living amenities will be located within the structure itself although it is not uncommon for a specific community of townhouses to share neighborhood expenses such as environmental upkeep or a security service.
These properties are considered to be extremely valuable pieces of real estate, as their prices will tend to rise over time. In fact, many of the most reputable urban centers are associated with townhouses. Thus, such structures are lucrative from either an investment or a rental point of view.