During Bitcoin Wednesday on 7 February 2018, Gabriel Kurman, co-founder of RSK (or Rootstock), explained how his team will introduce Ethereum smart contract functionality to Bitcoin.
He summarized the project this way:
When we started to build RSK, we realized that the only alternative that we had was Ethereum, but for many reasons, we thought that Ethereum might not be able to scale and that Bitcoin is not optimal when it comes to flexibility. So we decided to join the best of both; the flexibility of the Ethereum virtual machine with the security of the Bitcoin hashing power and the Bitcoin network to create RSK. So, basically, RSK is a business logic layer smart contract that can be secured and executed by the Bitcoin network.