The "successor" of Concurrent Versioning System (CVS) that has been built from ground up. It's currently in alpha-grade state (though it's already self-hosting!). It is distributed under the Apache license.

Subversion is, like CVS, a version control system - that is, it maintains a directory and subdirectories of files and keeps track of changes to each from different users. It's mainly intended for enabling versioning and collaborative authoring of program code.

CVS is nice, but it has some minorly inconvenient sides - like the difficulty of file renaming and slowness when working on large trees.

Subversion supports every feature CVS has. It also maintains the changes to directories, not just files - for example, renaming a file needs "remove and re-add" in CVS, which is sort of painful. The system is ACID proof and so changes are atomic - This is interesting because this way, a single change to multiple files looks like one change and not several separate changes to multiple files. (A simple explanation, from the Subversion book: CVS has "revision 1.2 of file foo.c", SVN has "file foo.c as it appears in revision 2 of the tree".) It has been designed to take better advantage of the client/server model from the beginning. Also, I18N has been thought of - the client programs can be translated, and the change comments and file contents can be maintained in different languages. Finally, the program uses the standardized WebDAV protocol for its network traffic.

Branching and comparing source code trees is pretty easy and efficient. In fact, SVN's idea of creating release tags is more along the lines of "copy the trunk/ directory to brances/release-2.0/".

The subversion trees are also MIMEtype-aware (if you check in a binary file, subversion will not mess around with it) and supports file properties (both arbitrary, like comments, and specified, like the MIME type).

The home page of the project, at the time of writing, is at <http://subversion.tigris.org/>.

Subversion has been working decently for a while now, and it has also hit 1.0 too. From a cursory glance, it ideed seems to be CVS Minus Suckiness Plus Good Stuff. =)