There are multiple possibilities with Android 4.2. The "board methods" are, as already explained in other answers: Every user has to install each app he wants to use (aside from the pre-installed ones) -- but they will be downloaded only once. Now, if one user updates the app, it will be updated for the other user(s) as well (which might get a bit problematic if one user does not want a certain app to be updated -- but that's up to the users to make clear).
If root is available on the device, some other interesting options pop up -- for which you might want to take a look at:
How to migrate applications to other user account without downloading the applications again?
With root, you can share apps (not data, of course) among users without the need of re-installation using Multi-User App Share, which even is reported to (partially?) work with paid ones.