This is where all of the computer components are stored. This is the main "show piece" since this is what people see. Recently cases have been incorporating more glass panels so that PC builders can show off their computer components. LED light strips and illuminated fans also aid in showing off compouter components. The physical size and amount of features of cases widely vary depending on the computer's main usage. The form factors mostly are categorized with the motherboards that are compatible with them.
ATX Full-Tower - These are the largest form factor and typically have the most amount of internal drive bay and storage space compared to the rest of the form factors.
ATX Mid-Tower - This case is a step down from the ATX Full Tower and takes up less physical space while maintaining a respectable amount of component storage.
Micro-ATX Tower - This case is extremely similar to the ATX Mid-Tower form factor. The main difference between these two are in compatibilty. This case is compatible with only the Micro-ATX form factor, whereas the the ATX-Mid Tower has support for ATX motherboards.
Mini-ITX Tower - This form factor supports the smallest motherboard form factor, Mini-ITX, while maintaining some support for additional storage and graphics cards.
HTPC - This is the smallest form factor, also known as Slim Form Factor. Home Theater Personal Computers are primarily about the same size as a cable box or disc player typically found on a TV stand in the living room. This form factor is the most barren, sacrificing large graphic card support, cpu cooler height, and expansion slots. These sacrifices are made so that the computer can achieve the most slim profile.