Hard drives or hard disks, are the only component in a computer currently that have storage without electricity. Although the BIOS in the computer maintains information it still uses the battery on the motherboard to do so.
Without a hard disk in a computer, anytime the power would flicker, or you would have to reboot, you would lose everything. A hard drive contains a large amount of storage where any other software is installed to. When you open a program or file, it is usually read from a hard disk then held within RAM.
A hard disk stores information like a tape drive, all the information is magnetically stored. The inside of the drive contains what looks like several compact discs stacked on top of each other with a small brush riding on each side of each disc. This brush is moved back and forth to get to any part of the disc as it spins.
Solid State Drives are the latest thing, there is no question, they are fast. These drives do not have any moving parts. They work more like RAM, except when you turn the power off, they hold onto their information. If you really want a fast computer, as of today, I still would not recommend getting one for the following reasons,
TIP! If you want a solid state drive for the speed, there is an alternative, the sata raid. This is my current personal choice.
When I buy a hard disk, the first thing I look for is brand name, my first choice is Western Digital, my second choice is Maxtor. Then I just choose the fastest drive that let's me stay in my budget. I also make sure I read the reviews about that drive so I only buy models people have success with.
For more information on benchmarking and hard disk reviews, go to both sections on the How To Build A Computer page.