Apple's OS X Yosemite is version 10.10 of the Mac operating system released on October 2014. With a modified look and many new features including Continuity (mobile device integration) and iCloud Drive (cloud integration), it is available to all Mac users free of charge.
Download OS X (Yosemite) from Apple. (*Within Mac App Store)
-- OS X (Yosemite): Upgrade Checklist
OS X (Yosemite): What's New
OS X (Yosemite): Support
OS X (Yosemite): Communities
Referenced from: Apple.com