Q: what is OSX Yosemite

Yosemite is the upcoming version of OS X. It will be released later this year.

It’s the next Mac OS X release. Wait until it officially comes out; when it does, it’ll probably appear in the Mac App Store.

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See:  http://www.apple.com/osx/preview/

Yosemite is only available for developers and Beta testers right now: You cannot install it on a Mac that is needed for your main work:   OS X Beta Program FAQ

I get that but I have error messages telling me iCloud won't store documents any longer unless I have Yosemite!

I get that but I have error messages telling me iCloud won't store documents any longer unless I have Yosemite!

Read the message again. It says something different.

If you upgraded your iCloud account to iCloud drive on one of your iOS devices, your documents will no longer sync with your Mac, unless you upgrade the operating system to Yosemity.  You can store in iCloud, but it will not sync.

See this help document:   iCloud Drive FAQ 

and this warning:   Be aware, be careful, be prepared for iCloud Drive | TUAW: Apple news, reviews and how-tos since 2004