Many years ago we were very limited to the type of computer case available in the market. Typically it was a simple dull box with a few simple buttons and outputs in the back. Nowadays they come in many shapes, sizes and colors.
When purchasing your computer, a casing is overlooked and people generally buy based on cost alone, a cheap case will normally be enough for a basic system. So its recommended to look into the casing and your requirements more.
When looking for your desktop, here are some of the typically shapes and sizes that you will come across.
The desk top design is more common with older computers, but now a day not a common design as its foot print is too large for an office desk. You are able to fit 2 or even 3 tower casing in the same area as one desktop. Due to the limited appeal the choice of shape and size is very limited so it is recommended to look for a tower computer casing.
A tower casing is by far the most popular casing in the market and come in three general sizes, the mini, midi and full size towers.
A mini tower generally packs less hardware inside compared to other sizes and due to small size can possibly over heat. So unless you are restricted for space then a larger version of the tower is recommended.
A midi tower is a popular size and it will be taller than the other casings but width and depth not vary much and therefore hold more hardware, this model is your typically home or even office computer.
The full tower is the largest version and quite often large and are used for more heavy use office computers such as servers and file sharing computers.