A common misconception that a computing novice makes is the thought that "Disk Cleanup" and "Disk Defragmenter" perform the same functions. Although you need both to keep your computer in good shape, each is used for different reasons.
What Disk Defragmenter Does
Disk Defragmenter organizes the file structure of your hard disk by gathering files that are stored in fragments and piecing them together to form a larger fragment (if possible, a single fragment). This is done by moving chunks of data from each file around until the file is a single large chunk.
How Disk Defragmenter Benefits Drives
Disk Defragmenter makes your drive function more quickly and reduces the amount of hops it has to take to read a single file. If the drive is strongly fragmented, Disk Defragmenter will likely improve drive performance dramatically.
What Disk Cleanup Does
Disk Cleanup is a utility for getting rid of old temporary files that are not very crucial for the functioning of your computer. This is done by emptying the Recycle Bin, temporary Internet files, and other places it sees fit to erase files, based on your choices.
How Disk Cleanup Benefits Your Drive
Disk Cleanup helps free up some space, giving you a little more breathing room. This is particularly beneficial if you filled up your computer with many temporary files from browsing the Internet or moved a lot of files to the Recycle Bin.
Optimizing Drive Performance
Using Disk Defragmenter and Disk Cleanup in conjunction can make your hard drive cleaner and operate faster.