Steem (or the Steem Blockchain) is the blockchain on which Steemit.com and busy.org interfaces with.
It is also the name of the native coin in the Blockchain. From an investor/trader point of view, Steem (STEEM) and Steem Dollars (SBD) are two tradable tokens on most major altcoin exchangers.
if you have any questions just ask away! i'll try to answer.