Disk Cleanup lets you easily remove all the little extra files that your computer stores...such as cookies/temporary internet files, setup files, thumbnails, etc. that can accumulate over time and take up space. For example, after a month of spending time on your computer you may have just a few MB of files that can be cleaned up like this....or you could have hundred of MB that you could clean up.
Disk Defragment essentially moves files around on the HDD/SSD so that the use of space being taken up is more efficient, allowing the files and programs to be accessed faster and decreasing the risk of hard drive failure from bad sectors. Over time your HDD/SSD will become fragmented, especially if you are adding and removing many or larger files/programs.