Steem coin is the cryptocurrency of Steemit — a blockchain-based social media platform founded in 2014 by Dan Larimer (the CEO of BitShares).
You can consider Steemit as similar to Reddit, where users can create and publish their content online and get rewarded for it. However, there is one major difference.
Unlike any other social network, Steemit is based on the blockchain, which allows it to be run in a decentralized manner. This means there is no censorship, no central authority, no downtime, no data vulnerability, and no data abuse. It solves a seemingly-endless list of problems seen in centralized social platforms (i.e. Facebook, Twitter, etc.).
As well as the above unique advantages, it also rewards its users using cryptocurrency! So, rather than post to social media, while the central party (Facebook, Twitter, etc.) cash in on ad revenue, you can post to social media and be paid for it yourself.
The Steemit blockchain actually has three different cryptocurrencies on its blockchain!