"As a concept the Rootstock  platform is one of those ideas that once it is proposed it is obvious that it is a great idea. Essentially Rootstock aims to be what Ethereum is, a decentralized, Turing-complete smart contract platform. However, Rootstock aims to utilize the Bitcoin ecosystem rather than creating a new one from scratch. The way this will be accomplished is via the still not fully implemented sidechains technology . This approach presents both advantages, and its own set of challenges. We will briefly outline what the Rootstock platform promises to offer and then will discuss the main points of the proposal."
"Rootstock will exist as a Bitcoin sidechain. If you haven’t heard of the sidechains proposal you can review the whitepaper here, or read our explanation of sidechains here. But in summary a sidechain is a blockchain that is separate from the main Bitcoin Blockchain, but assets can be transferred back and forth."
Full post here by Albert Szmigielski