There are three tokens used on the Steemit network.
Steem (STEEM) - Steem is the base currency of the Steemit network. Think of it like an inflationary fiat currency. Right now, Steem inflates at 9.5% per year. However, the inflation rate will decline by approximately .5% per year. The inflationary nature of Steem means it is probably not the best of the three to invest in.

Steem Dollars (SBD) - Steem Dollars are digital tokens pegged to the U.S. Dollar. The founders of Steem wanted to pay content providers using a token that didn't have wild value swings. Holders of Steem Dollars recieve an "interest rate" equivalent to 3%. That means 3% of all newly created Steem goes to the holders of Steem Dollars. Generally Steem Dollars should be trading at or near $1 USD

Steem Power (SP) - Steem Power gives you the equivalent of an ownership stake in the Steemit network. 15% of all new Steem created is given to holders of Steem Power. So if you owned 100 SP on Jan 1, 2017 you will own 115 SP on Jan 1, 2018. Steem Power holders receive the biggest rewards and have the most power on the Steemit network. The more SP you have, the more weight your upvotes and downvotes carry. If Steemit becomes the next Facebook, then early investors in SP could become rich...

By the way the guy that created the Steemit Blockchain is also on the EOS Team I would check out EOS.