This past April, Apple surprised us with an announcement that the general public would soon be able to access OS X betas. The reason for the surprise is the simple fact that Apple's forever been a conservative company, careful about what winds up in its customer's hands. So to give them not only beta software, but a beta of the actual OS, is rather huge.
Well, that first beta comes today, and it's based on a developer build that came out Monday. Anyone who has signed up for the beta program in the past should be receiving an email at some point today; if you're one of those who hasn't signed up, you can do so on the official beta page.
In order to use the beta, your Mac will need to be up-to-date with Mavericks. Beyond that, Apple highly recommends that you install this beta on a second machine, just in case problematic bugs exist. Regardless of where it's set to be installed, it's also recommended that you use Time Machine to back things up.
While Yosemite will bring some specific features that tie in with iOS 8, none of those are going to be present in this beta - something to be expected on account of the fact that iOS 8 isn't due for another couple of months.
If you happen to give the beta a test drive, be sure to leave your comments to let us know what you think.