Hi,
Your final mix or master will be one stereo file, whereas stems would be a number of stereo files making up the same thing.
You might give a mastering guy stems which are mixed down stereo wavs of your drums, rhythms, vocals, and extras, for example.
He wouldn't be able to manipulate each component of the kit, for example, but he can work on it as a group.
I have a guy who tracks vocals overseas. While I'll be doing the final mix, I generally send him stems to track vocals to so that he can raise/lower components of the mix to suit his headphone monitoring requirements.
It saves the hassle of sharing the whole session, but gives him enough control for his needs.
I haven't really used garageband much but I'd hope, like any daw, it would have a folder containing all the necessary parts for your session.
Being that it's apple they might even use some self contained package or library, like iPhoto libraries etc.
I can investigate and find out for sure but in the mean time have a look to see where your sessions are stored and see if what I'm describing is the case.
If so, all you have to do is save your session and quit, find the relevant folder/package, right click and compress it, then take/move/copy/send/share the compressed zip with your friend.
If you don't have a file sharing service, wetransfer is very user friendly and no sign up is required.
Hope that helps.