Defragmentation
Defragmentation describes reorganizing a hard drive's data to help increase the proficiency of accessing the data and prevent file fragmentation. After the hard drive data is organized, the computer's overall speed will seem faster, and it'll take less time to run a program and open files.
How to defrag
A hard drive can be defragged with a defragmentation utility, such as Microsoft Windows Microsoft's Defrag. Another great utility to keep your hard drive clean is Diskeeper by Diskeeper.
If your computer has an SSD (Solid-State Drive), it does not need to be defragged.
Why does defragmentation speed up the computer?
A mechanical hard drive with a read/write actuator arm must move to a data location on the platter to read and write data. If that data is scattered around the platter, it takes longer to access it. However, if all of the data is in the same location, it can be accessed faster.