Steemit is a blogging platform that pays you. Not only can you be paid for each post but your comments, as well as, your upvotes, too.
Steem is a cyrptocurrency, like Bitcoin. But, whereas Bitcoin is a mining platform, Steem has taken the cryptocurrency module and applied it to a social media platform.
By creating posts & comments and upvoting on good content you are essentially working to help create Steem and thus juicy rewards are distributed to you for your efforts, in the form of Steem Dollars and Steem Power.