OS X has supported iCloud Documents for a while now, which let OS X apps share documents with other Macs and iOS devices as long as you have the same apps on them. But you can't easily see all your documents, and moving documents between the Mac's Finder and iCloud Documents is unintuitive and awkward.
In Yosemite, iCloud Drive makes these documents visible in the Finder and in Open and Save dialogs to all apps (not only the ones from the Mac App Store) like any other drive, similar to using a Box, Dropbox, Google Drive, or OneDrive virtual drive.
You set up which apps participate in iCloud Drive in the iCloud system preference for live syncing as you work. Otherwise, syncing happens after you save.
Note that iCloud Documents hasn't quite gone away; individual apps still save to their specific folders by default. However, those folders are now visible in iCloud Drive.
Also, the Mail app can use iCloud to automatically store large attachments (that's the MailDrop setting for each account), so they're not clogging up your servers. Recipients get a link to the file and don't need an iCloud account.