It's basically something like Medium, Reddit, Tumblr, or even Facebook with one MEGA difference:

Steemit pays both the content creators when their work gets upvoted, as well as the people who curate the best content on the site by upvoting others work.

This is possible because the Steemit platform is built on top of a new kind of digital currency called Steem.

Every day, Steems are created by the network and distributed to its users, who can exchange these digital currency units for actual real money.