Wherever people live, either on their own, with a few others, or more and more people until you have thousands of neighbors, there are a variety of different classifications of habitations. The explanations below begin with the smallest and work their way up to the largest.
Often a single dwelling can be found in the wilderness due to a person's need for personal space. Although not only hermits occupy single dwellings, they are the most numerous residents. A single dwelling can be occupied by one person, or possibly a couple of families, but it must remain as one building for living quarters (other buildings for other reasons can exist). Lighthouses and large farms often qualify in this category as well.
A hamlet is a collection of dwellings, and a couple of other buildings, where the population is greater than that of a Single Dwelling, but does not exceed 80 people. Hamlets tend to be the most numerous habitation types.
A village is a collection of dwellings, and a couple of other buildings, where the population is greater than 81 and does not exceed 200 people.
A town is a collection of dwellings, and several other buildings, where the population is more than 200, but does not exceed 1500 people.
A city is a collection of dwellings and associated buildings where the population is more than 1500 people.
Usually not a permanent settlement, some encampments are stationary long enough to fall into a "habitation" category. These do not have to be military in origin, but must have a large number of people occupying the encampment.
A single stone tower, whose purpose is to re-supply troops, and provide a permanent base, observation outpost, and military reminder to the area it is in. Often, small villages can spring up around or very close to a Keep.
Made of stone, a fortress is most often a military base, built to withstand attack from opposing armies. They can be small, consisting of a stone wall protecting a few buildings within, or can be large enough to contain a town. Castles would also fall under this categorization.
Similar to a fortress, a stronghold is usually built into a mountain, making use of natural caves which may or may not be expanded or modified.