Subversion is built on a portability layer called APR (the Apache Portable Runtime library). This means Subversion should work on any operating system that the Apache httpd server runs on: Windows, Linux, all flavors of BSD, Mac OS X, Netware, and others.
The easiest way to get Subversion is to download a binary package built for your operating system. Subversion's website (http://subversion.tigris.org) often has these packages available for download, posted by volunteers. The site usually contains graphical installer packages for users of Microsoft operating systems. If you run a Unix-like operating system, you can use your system's native package distribution system (RPMs, DEBs, the ports tree, etc.) to get Subversion.
Alternately, you can build Subversion directly from source code. From the Subversion website, download the latest source-code release. After unpacking it, follow the instructions in the INSTALL file to build it. Note that a released source package contains everything you need to build a command-line client capable of talking to a remote repository (in particular, the apr, apr-util, and neon libraries). But optional portions of Subversion have many other dependencies, such as Berkeley DB and possibly Apache httpd. If you want to do a complete build, make sure you have all of the packages documented in the INSTALL file. If you plan to work on Subversion itself, you can use your client program to grab the latest, bleeding-edge source code. This is documented in the section called “Get the Source Code”.