If we group and classify a number of settlements according to their size and shape, the result is settlement hierarchy.
As you move up the hierarchy, the size of the settlement and the distance between similar sized settlements increases. As you can see from the diagram above, there are more cities than conurbations, more towns than cities and more villages than towns.
The number of services that a settlement provides increases with settlement size.
Small settlements will only provide low-order services such as a post offices, doctors and newsagents. Large towns, cities and conurbations will provide low and high-order services such as leisure centres, chain stores and hospitals.
Larger settlements and conurbations have a much larger sphere of influence than smaller ones. This means they attract people from a wider area because of the facilities they offer. Cities such as London have a global sphere of influence, whereas a small hamlet or village may only have a sphere of influence of a couple of kilometres.
Services such as department stores selling high order goods have a higher threshold than those selling low order goods such as newsagents. This means they need a higher number of people to support them and make them profitable, therefore they will only be found in larger settlements. It also means that there are fewer big department stores than small newsagents.
The range of a service or product is the maximum distance people are prepared to travel to purchase it. The range of a newspaper is much lower than an item of furniture for example.