Steem is of course much more complex than a single sentence could handle, but let's just go with whatever comes to your mind, it doesn't have to be right, just go with your flow.
If I had to describe Steem in one sentence:
Steem is a disruptive platform & experiment on how a decentralised group of people can utilize & distribute a shared resource pool to grow and enhance said platform/currency.
What's yours?
50% of rewards are being burned to @null