The goal of the Subversion project is to build a version control system that is a compelling replacement for CVS in the open source community. The software is released under an Apache/BSD-style open source license.

• Most current CVS features.

  Subversion is meant to be a better CVS, so it has most of CVS's features. Generally, Subversion's interface to a particular feature is similar to CVS's, except where there's a compelling reason to do otherwise.

• Directories, renames, and file meta-data are versioned.

  Lack of these features is one of the most common complaints against CVS. Subversion versions not only file contents and file existence, but also directories, copies, and renames. It also allows arbitrary metadata ("properties") to be versioned along with any file or directory, and provides a mechanism for versioning the `execute' permission flag on files.

• Commits are truly atomic.

  No part of a commit takes effect until the entire commit has succeeded. Revision numbers are per-commit, not per-file; log messages are attached to the revision, not stored redundantly as in CVS.

• Apache network server option, with WebDAV/DeltaV protocol.

  Subversion can use the HTTP-based WebDAV/DeltaV protocol for network communications, and the Apache web server to provide repository-side network service. This gives Subversion an advantage over CVS in interoperability, and provides various key features for free: authentication, wire compression, and basic repository browsing.

• Standalone server option.

  Subversion also offers a standalone server option using a custom protocol (not everyone wants to run Apache 2.x). The standalone server can run as an inetd service, or in daemon mode, and offers basic authentication and authorization. It can also be tunnelled over ssh.

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TortoiseSVN is a really easy to use Revision control / version control / source control software for Windows. It is based on Subversion. TortoiseSVN provides a nice and easy user interface for Subversion.
It is developed under the GPL. Which means it is completely free, including the source code. But just in case you don't know the GPL too well: you can use TortoiseSVN to develop commercial applications or just use it in your company without any restrictions.
Since it's not an integration for a specific IDE like Visual Studio, Eclipse or others, you can use it with whatever development tools you like.

As a Subversion client, TortoiseSVN has all the features of Subversion itself, including:

• Most current CVS features.
• Directories, renames, and file meta-data are versioned.
• Commits are truly atomic.
• Branching and tagging are cheap (constant time) operations.
• Efficient handling of binary files.