With mafia now supporting an internal svn client, some scripters are undoubtedly wondering what "svn" is, how it works, and what it can do.
What it is
Subversion ("svn") is a centralized version control system. A repository ("repo") exists at a url - this repo contains your project. It not only contains the files for your project, but every previous version of those files that's ever existed. Think of old versions like hitting ctrl-z on a Word document - you can revert changes all the way back to the start, or anywhere in between.
It is more than just a place to save files though. There is something called a Working Copy, which is a local version of a remote repository, plus any local changes you may have made. The process of creating this Working Copy is referred to as "checking out" the repo, or "checkout." You have probably also heard the term "commit", which is simply taking some/all local changes in a working copy and syncing them with the repo. Then other users sync their working copies with the repo, and voila!
How it works
A working copy contains a hidden folder, .svn, that contains all of the metadata needed to know how your local version differs from the repository's version. This is created for you when you checkout the project. You don't really need to know any of these details, really, for the most part using svn does not require knowing how it works under the hood. What you may be interested in is how to create your own repo.
I'll go over the easiest method under Windows - using TortoiseSVN. Under *nix you should probably just be using the command line "svn" client, though I imagine there are a million different graphical alternatives. There are alternatives in windows as well, I just prefer TortoiseSVN for its file system integration.
First, get a sourceforge account. Make a new project there. Download TortoiseSVN and install it. Make a new folder somewhere on your computer. Right-click, and choose SVN Checkout... from the context menu. You will want to give it the address that you see on your new sourceforge project code page - for now just grab the https:// address, it's easier. Ignore the commands in front of the address and after it, those are for command-line svn tools.
The rest of the default options are okay. Click OK and you should get a new working copy of your repo! Cool. Open it up and let's add some stuff. Add a new directory (call it whatever), within that directory add a folder called scripts/, and within scripts/ put a new text file. Now navigate back up to the root of your working copy, right click on the folder, and choose SVN commit... Tick all the files/folders (tick the "show unversioned files" box if it's not ticked), give it a quick commit comment up top. You should have to enter your sourceforge user information, save it if you want. If you've done everything correctly, your new project should have been committed.
Optional: to really play around with this, checkout a second working copy somewhere in another folder on your computer. Try committing changes from one working copy to the repo, then doing right-click SVN Update on the other working copy. Notice how TortoiseSVN helpfully changes the little file/folder icons for you when you make modifications to things. Hopefully you're starting to see how this whole thing works.
What it can do
svn is a very mature tool by now, so there's an absolute ton of stuff that I won't cover. You may for example notice that mafia itself has a "Latest SVN Changes" forum, which has .diffs of all the changes to files within mafia. There's also creating patches, merging repos, yada yada... Won't talk about it.
I'm more interested in talking about how mafia svn integration works. Doing svn checkout <repo> creates a working copy of the repo in your svn/ folder. Then, mafia "pushes" the contents of that working copy to the appropriate subfolders. For example, if a repo has a scripts/dostuff.ash file, that file gets copied to your scripts/ folder.
svn update goes through all your working copies and performs an update. There are other operations that can happen here. With a checkout, you can only be adding ("A") new files to a working copy. With an update you can update ("U"), merge ("G"), delete ("D") or also add ("A") files. Here, we warn the user whenever a file is added, due to security concerns. Note that "U"pdating only happens if the "pushed" local file still exists in the user's directory. I am bolding this because I'm sure it will confuse some people. This feature is a safeguard against svn re-installing scripts over and over that the user just wants to have deleted.
If you installed TortoiseSVN above and browse to the svn/ folder, you may notice something: the working copies checked out with svn checkout are fully-featured working copies, just like you checked out using TortoiseSVN. This means that you can make changes directly to this folder and commit them to the repo using TortoiseSVN - if you have repo commit permissions, of course.
And yeah. I meant this as a "quick" primer and it turned into a "moderately long" one, but them's the breaks.