They are distributed unter a Creative Commons 3.o share alike license. This means that you can download and use them as desktops, to illustrate your blog entries, to make your presentations more interesting or for whatever other purpose there is. The only thing you should not do is sell them as your own work. You may modify them if you are willing to share them for free too.
If you have any questions, feel free to contact me at email@example.com or write your comments directly on this site.