The outer shell of the tower is called the case.
It is what holds the computer components in it.
Think of the case as being like skin.
It's what holds everything together in your body.
The case is also the place where you will find the power supply, the extremely important fan that keeps the computer cool, and the various buttons on the front.
The fan is very important to a computer because it will prevent overheating.
When you computer overheats it will shut down or very possibly breakdown.
Many of today's powerhouse gaming computers will have two or three fans on them.
These are needed because they have numerous devices on them all generating heat.
You will find many large rectangular holes in the case.
This is completely normal, as these holes are actually drive bays.
The purpose of drive bays are to allow you to add new components to your computer.
A few of the components that you might be adding are a CD-ROM, a DVD player or a removable-media drive.
Some of the newer cases will have a hinge on it that will let you swing open a door that will allow you access to the drive bays.
However, many times you will find that most cases, you can just cover them with a plastic fitting that matches your case exactly, effectively hiding the hole.
There are many different sizes of cases on the market today.
The size that you depends on the type of computer user that you are.
If you do nothing but surf the web and send emails then you are not going to need a large case.
If you are a graphics design professional or a gamer then chances are you will need a larger case.
This is because you will need more devices and fans on your computer.
So before you get a new case make sure you understand what your needs are so you don't buy one that you don't need.