You're going to be making major changes to your Mac: installing new system files, deleting old ones, applying for new permissions, and resetting preferences.
OS X Mavericks (version 10.9) is the tenth major release of OS X (now named mac OS), Apple Inc.'s desktop and server operating system for Macintosh computers.
Before you install Yosemite be sure to check you have the right Mac.
It’s fairly common for computer users to delay updating to major operating system releases, and though Mac owners tend to be a bit better at upgrading than Windows users, many are still running old versions of OS X.
This article is aimed at the procrastinators (and there’s a surprisingly large amount of them – nearly 17% of Mac users are on Lion and another 20% on Mountain Lion), particularly individuals with Macs still running OS X Lion, in any version from 10.7 through 10.7.5. OS X Mavericks is free from the App Store and simple to install.
The entire process is usually completed in an hour or so for a simple upgrade, or you can clean install if that’s your preference too.
To put it simply, if your Mac can run OS X Mavericks, it should have no difficulty with OS X Yosemite.
You can find a detailed list of which Macs will be supported in the guide: OS X Yosemite Minimum Requirements Once you're sure that your Mac meets the minimum requirements, you're almost ready to proceed, but there are still a few more steps to go through to ensure that your expectations will be met by Yosemite.