SVN stands for Subversion.
It is a tool that helps developers work on the same set of source codes without having to download the package when one single difference was made... really safes a heck of a lot of bandwidth... believe me...
It is also very useful if you make a small change and you want to contribute that change... that small change is what is usually referred to as a patch... Very nifty...
When you want the source of a project, you need to identify the URL and do a "checkout", this will then download all the files that are within that "repository". After you downloaded that initial checkout... you only have to update your repository with the URL provided (You don't need to remember the URL, SVN does that for you... cool hey!), SVN will "see" what changes was made to the server and update you, it will also "see" any changes you made so that if you screwed up somewhere, you can "revert" to the original on the server... even cooler! (I screwup allot you see!
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I recommend Tortoise SVN... integrates with Explorer of Windows (Don't know about Linux though, normal command SVN might be sufficient)
A small brief intro to SVN... I hope...