Logged in just now and found my reputation had dropped 10 points with the legends 'removed' and 'user was removed' yet I can login.
What does this mean? I've looked in the faqs but found nothing pertinent.
What have I done?
You haven't done anything. Some user who voted for you had their account deleted, either by their request or due to some other factor. This results in a loss of reputation earned from that account.
If it's any consolation, it just happened to me, too. :-(
The details of user deletion are covered at the page where you ask for user deletion: https://apple.stackexchange.com/help/user-deletion
So - unless you were the only time that user voted on anything, this account would have been deleted by Stack Exchange in response to the procedure listed above. Initially I was disappointed when I first noticed I had lost reputation points on this, but after seeing it happen several times, it makes sense to prevent someone from making an account - voting up (or down) a particular user and then leaving. When someone exits - their down votes which took reputation from others are reversed as well as their up votes leaving the voting system as if they never showed up in the first place.
It is apparently the logic of those who make these decisions that the site grants the ability to vote only after you get a vote from someone else and also requires you to not delete your account to keep your votes in play.