You may find this link helpful...
http://www.ufopaedia.org/index.php?title=Difficulty_(EU2012)
Actually that wiki has great information for most aspects of XCOM, although it's still being filled in on some areas. For example, it doesn't mention that Thin Men get 3 HP on Normal, 4 on Classic, and 6 on Impossible (and some other differences in HP for units along those lines).
AFAIK, the "unshackled AI" just means it's willing to try to fight you with more than 5 aliens at once. On Normal, if you trigger more than 5 aliens, the excess are supposed to just back off and wait for their friends to die before stepping in. You can
and will get swarmed on Classic/Impossible.