Computer hardware is comprised of several different components which are required in order for someone to actually be able to use a computer.  The computer box that houses these different components is what protects the parts inside.   There is also a fan inside the box that helps to keep the components from overheating and to keep dust and dirt out.  A supply of power is also located inside the box.  These components perform many different functions.  These functions include the temporary storage of data, the permanent storage of data, manipulation of data, and the interface to outside components.
When you open the box you will see inside that there is one component that ties everything together.  This is called the motherboard.  This allows all the pieces of the computer to receive power and communicate with each other. The motherboard holds several subcomponents.  These consist of memory which is what is used to provide temporary storage of data, a microprocessor which is used to process the data, video and sound cards, and one or more permanent storage devices, i.e. hard disk, CDROM, DVD drive, etc.
There are some components of a computer that can be added later or are removable.  These are called Removable Media Devices.  For example, if your current computer did not come with a DVD drive, you can purchase one separately and connect it to the computer with a USB cable.  Another type of device that can be used externally is a tape drive which would be used to back up all your data on some regular basis.  The flash drive is a very useful component.  This can be used to quickly remove or copy files from your computer and then plugged into another computer to access those files. 
To learn more details about computer hardware and its purpose it might be a good idea to take a class or purchase a book.  If you’re not into the classroom scene, you can always look online for an introductory class.  Most of the community colleges offer these courses and can be completed at your own pace.  If you don’t have a computer yet, there is usually a computer laboratory that students can utilize.  The parks and recreation programs in your area often offer introductory courses and usually at quite a reasonable price.
If you are in the market for a computer, your options are nearly endless.  Keep in mind what your main use will be for the computer.  For example, if you are going to use it specifically for checking email and an occasional game of solitaire, you probably don’t need the largest and most expensive computer on the market.  On the other hand if you are planning on using the computer for gaming and heavy internet use, you will need something completely different.  It is important to talk to people to find out if they use their computer for similar needs, as well as, finding out if they are satisfied with their computer or what they would recommend.