The post I linked to first is a good start.
Here's how I understand it:
STEEM: The Steem cryptocurrency
Steem: The Steem blockchain (or the code, protocol, or running mainnet, depending on context)
Steemit: One of two things:
Steemit, inc: The company who launched Steem and primarily develops the Steem protocol and owns...
Steemit.com: The primary browser of the Steem blockchain. There are many others, but most conflate Steemit.com with Steem, when really they are different. Steemit.com is just a centralized website run by a centralized company using a decentralized blockchain (of which they do happen to have a large, centralized stake of tokens which apparently they just sold to the Tron foundation).