Victorian architecture is a fairly complex topic. The reason is because different countries have different forms of Victorian architecture. So when speaking about it in England, it is different than when you speak of Victorian architecture in the United States.
France and Italy also had their own variations of Victorian architecture. Generally though, the style derives from when Queen Victoria was reigning as monarch in England. Generally though, the actual style may extend a little bit further past her reign according to many architects.
Many of these Victorian style homes still stand today. It is actually still a style that many people will follow. What you find among most of the Victorian styles is that many of them use shingles. This was a new item which stems from the industrial revolution. The use of shingles frequently allowed for much more intricate roofs, although the Italian Victorian Style homes which you may find in America often had very simple and low roofs.
However, there was a common style that we still see today referred to as the Victorian Shingle Style architecture. These homes with this style often have several gables, use of shingles. Even lower floors on large homes will have separate overhangs over them which use shingles for the roofing.
There were a few forms of the Victorian architecture which did not feature shingles. You can look at both the Second Empire and Gothic Revival forms of Victorian architecture which may be seen more commonly around England than other countries.
These generally have sharp pointed roofs and much more intricate detail on them. They give a sense of height on them where they often extend far past the actual use of the building. Some say that the influence came from medieval buildings.
Then, you can not forget the much simpler styles of Victorian homes. The stick architecture as well as the Folk Victorian style are both relatively simple homes. The Folk Victorian primarily used trim and things which were in mass production throughout the industrial revolution and continue to be today.
However, the stick architecture were generally fairly simple architecture which used timber throughout the exterior to give a more custom feel to it. They were frequently quite square with no gables.