Itch.io provides download links which are the same thing as keys (like on Steam). These require an itch.io account if you do not have one already (so that these keys can be made available to you alone).
That download code is unique to your email and can only be used by you once claimed. I believe once you download something, you've "activated the key" and it should then be associated with your account.
As for Steam keys, for Kickstarter backers, I will definitely be providing Steam keys once they become available (sometime in March). I haven't yet decided if they will be made available for non backers.
Also, I am working on the Mac build as well, it just requires more Q/A on my end, but it will be made available soon at no extra cost.
I will make announcements as each of these things become available in the future. Thanks for the questions everyone, keep em coming!