Rootstock is a project to bring smart contracts (Turing-complete smart contracts) to the ecosystem of bitcoin. Rootstock (www.rsk.co) is a separate blockchain attached to the bitcoin blockchain through the use of a two-way peg. When you send a bitcoin over to Rootstock (via a special address where it will be locked and unspendable), it becomes a "smart" bitcoin living in Rootstock, which can be sent back to the bitcoin chain. This concept is known as a sidechain. Technically, Rootstock nodes are using a port of Ethereum (Java source), harnessing the EVM  and its Turing-complete language to write and run smart contracts. Moreover, it promises faster payment and scaling up to 100 transactions per second.