Defragmentation

What is fragmentation?

When data, such as a file, is stored on a hard drive the operating system attempts to store that file in one section, locations that are connecting without a break, space. When you have a new hard drive, storing data in contiguous spaces is not a problem. As you use the hard drive, though, files will be deleted from it and small pockets of space will be created on your hard drive. These small pockets of space on your hard drive is called fragmentation.

What is defragmentation?

Defragmentation is the process of locating the noncontiguous fragments of data into which a computer file may be divided as it is stored on a hard disk, and rearreging the fragments in sequential order, it "picks up" all the pieces of data that are spread across your hard drive and puts them back together

Defragmentation reduces data access time and allows storage to be used more efficiently.

When and where is it neccessary?

Some operative systems automatically defrag by a period of time,but others don't.

It is important to defrag your hard disk from time to time, so it works faster.

Windows 7 and Windows 8 automatically defragment your hard drives for you 
If you're on Windows XP, you'll need to defragment your drives yourself
If you use a Mac, the you probably don't need to manuallu defragment, since OS X will do it automatically for you.