Defragmentation

What basically is fragmentation?

Your drive is fragmented, when the files saved are saved in some fragments instead of being on one place.
Thing is simple just like, your sixty pages file. But the pages are scattered. One on the table and other sometimes below the table or even your bed. You can basically imagine a boy's messed up room. You know where are they, but it'll take some time to get full access.

Solution is to defragment.
Go to All Programs> Accessories> System Tool> Disk Defragmenter.

Or you can use external software. Like Disk Keeper or Tune Up Utilities.

TuneUp Drive Defrag