Steem.it is a blockchain-based social media platform where anyone can earn rewards by posting relevant content, curating quality content by upvoting and by holding Steem based currencies in a vest fund, which generates interest.
There are three main currencies in Steemit: Steem, Steem Power (SP) and Steem Backed Dollars (SBD).
Steem is liquid and can be bought in an exchange and converted into steem dollars or steem power.
Steem Power is basically Steem that is locked in a vesting fund for 3 months. Users can use steem power to upvote content and get curating rewards.
When a user upvotes content his steem power gets depleted and then slowly regenerated. Steem Power holders recieve interest from their holdings. The more Steem Power a voter has the more revenue he'll generate for himself (in form of SP) and for the content creator (In SBD).
There is also an incentive to upvote content early, as the rewards are distributed according to time. The earliest votes gets the biggest share of the reward.