Steem is the coins that powers the blockchain based platform Steemit. Steemit a social content network that bases its operation on the blockchain. The platform incentivizes content creators to make great contents by rewarding them with STEEM.
It uses its currency as a reward to writers when their content gets upvoted. Apart from the content creators themselves, readers also earn tokens as reward for organically engaging with and curating contents. Try to check out their whitepaper to understand their economic model and how the system works in detail.