PS4: How Digital Sharing Works

And it sounds pretty straightforward.

Sony has released a new FAQ detailing how digital game sharing will work on the PlayStation 4
The FAQ explains that you'll be asked to register one PlayStation 4 as your primary console. Doing so will mean anyone playing on that console on different accounts can play games and apps you've gotten from the PS Store and make use of PS Plus benefits, or you can connect to it over the internet with your Vita to make use of Remote Play.

Registering your PS4 as your primary system will allow other users who play on the same console with different accounts access to any games you've downloaded without you having to log in. Additionally, if you have PS Plus and register a PS4 as your primary console, then every account that plays on that system will be able to make use of certain benefits like online multiplayer even without their own PS Plus subscription.
If you want to play on another PS4 that isn't your primary console, you can download any game you've bought on the PS Store as long as you sign into PSN. Only you'll be able to start the games you've downloaded, and you can only be logged into two consoles at any one time: your primary PS4 and  one other secondary one (like a friend's). The good news is there's no limit on the number of systems you can download your games to. You have to be logged in in order to play, but from the sounds of things this means you can share digital games with as many mates as you like, though only one of these secondary consoles will be able to play a title at any given time and you'll have to tell them your password so they can log in.
As has been mentioned before, the situation with discs remains the same as ever. Sony has reiterated that, "You can share disc games with your friends and play used disc games on your PS4 system just like you can on a PlayStation 3 system. You do not have to connect to the Internet or pay any usage fees to play used games."

The PlayStation 4 will be available in North America on November 15 and in Europe on November 29.