It's about 12:46 am in 8/5/2013, I don’t know anything about Subversion, there are 6 open tab in my browser to discover what is Subversion for. I'll share it in an hour!
Subversion is all about managing and tracking the changes and overall it is an advanced version control. It's tools are so useful for versioning, changes of files and web pages and any document of the software. The main idea behind Subversion is to prevent programmers to work on the same file in group working. Subversion created in 2000 and it's tools are usually open source. Subversion is something different from continuous integration, because Subversion cares about the directories and files it is supposed to track the changes to.
What's the need for it?
Software development is an evolving progress. It could be changed several times in developing process, it could be divide into several versions and so on. All of this need to be keep stacking. As I mentioned, Subversion introduced to prevent programmers tp work on the same files and overwriting each other's codes. Subversion make the developers to be sure about Version control and don't wast any time about that.
You can see the useful resources I used to know about Subversion below