Subversion is an open source version control system, a successor of CVS. It is used to maintain current and historical versions of files such as source code, web pages, and documentation.
You upload your files (documents) on the server, and you need to commit your changes at the end of the day. It is accessible, secure, well ordered and easy to use, and good to archive far-from-the-computer. Every time you make a change on the server, it saves into a new version (Revision 1, 2, 3..).
I use SVN for keeping track of the modifications I make by working on my thesis from two computers - the one in the lab, and the one at home. Since I work 2 days per week from one, and 3 days from the other, I need to keep an updated versions of my files. It is similar to carrying your usb-stick, but much more secure (no chance to loose it), much more reliable (since it is saved on a secure server). It is faster, accessible from everywhere, and fun.