STEEM is the currency (along with SBD). Steem is the blockchain. Steemit is the oldest and, so far, most popular blogging app on Steem. Steemit Inc is the company that developed the blockchain and who runs Steemit.
I know you know all this. But there is a campaign, initiated by @taskmaster4450, to keep the nomenclature straight in order to not confuse newbies, which is I why I wish the older users would not be careless in their use of terminology.