Yosemite was first unveiled during June's World Wide Developer Conference (WWDC). During the “It's Been Too Long” media event earlier on Thursday, Apple said Yosemite would be released later in the day, and now is apparently later.
Yosemite is free. We have a handy list of Macs supporting Yosemite if you're looking to know whether or not you can run it.
[Update: Doug Grinsberg noted on our Facebook page that the release version of Yosemite is build 14A389—the last developer seed was build 14a388, also known as GMC3. The last public beta version was build 14A388b. That makes the MAS release newer than all other released builds.]