Mavericks is the marketing name given by Apple to their latest operating system, OS X 10.9.
Mavericks was released on 10/22/13, and is a free OS X upgrade to anyone that has a compatible Mac. It must be downloaded through the Mac App Store, and require entering an Apple ID to install.
If the Mac is currently running OS X 10.7 or 10.8, it should be compatible with Mavericks. Some Macs running OS X 10.6 are also supported. For a full compatibility list, and basic overview of the upgrade process, see http://www.apple.com/osx/how-to-upgrade/
As of this blog post, Apple has not released any OS updates to Mavericks, and it is still at OS X 10.9.0. As with previous Apple OS upgrades, Mavericks is experiencing major growing pains.