In addition to OS X 10.9.2, Apple has posted security updates for OS X 10.7 (Lion) and OS X 10.8 (Mountain Lion). Both of those are available either through the Software Update system preference or directly from Apple's web site.
While much has been made of a data security bug that affects iOS and OS X Mavericks involving SSL/TLS, Lion and Mountain Lion are not affected by that problem.
Despite that, Apple has patched over two dozen other security issues in these new updates. Apple has posted a long list of patches in 10.9.2 and the new updates worth looking at if you're curious about what's changed.
Many Mac users have upgraded to Mavericks, Apple's first free OS X upgrade since 10.1. But some users are still hanging on to older versions of OS X for compatibility's sake or because they neither want nor need the new features of Mavericks. If you fall into that camp, please make sure to apply this security update as soon as possible anyway - better to be safe than sorry.