Architecture is a large subject, and covers many different time periods, histories, and types of buildings. For the purpose of this brief writeup, I am talking about the difference between residential architecture in the United States of America, especially as it might be observed by someone bicycling through the streets, at a glance.
The early 20th century was one of the times that a definitive style of architecture arose in the United States, most famously led by Frank Lloyd Wright. Although this included many experimental features, it also included the Craftsman or Arts and Crafts style home. Also called bungalows or prairie homes, one of their most important features is the front porch, which allows people to have a semi-public place to converse with neighbors. Also, this being before the common adoption of automobiles, they do not have attached garages. (Garages are usually detached, or at most connected with a breezeway). This being a time before widespread automobile use, homes from this area are usually built closer together.
Two things happened: The Great Depression, and World War II, both of which paused new home building, as well as development of architecture style. Then World War II finished, the Baby Boom started, along with the widespread adoption of the automobile and suburbanization. Most postwar houses have two easily identifiable architecture features: they lack a porch and they have a built-in garage. They are also built further apart.
These two easy architectural features speak to a world of social difference: houses built before the war were embedded in some type of social matrix, where the house was built with features that oriented it towards the community, and presupposed the house as part of a social environment. Post-war, houses were built in isolation. People with television didn't need porches, and people with cars could treat the distance between their house, work and shopping as a vacuum.
Sometimes, when I was out bicycling around, I would be passing through a typical housing development, widely spaced ranch houses in suburban cul-de-sacs, and then come across a little cluster of craftsman homes, a memorial in amber from when the area was a crossroads in the middle of farmland, and from when American communities looked very different.