Stacks connects to Bitcoin, enabling you to build apps, smart contracts, and digital assets that are integrated with Bitcoin's security, capital, and network. Stacks is a blockchain linked to Bitcoin by its consensus mechanism that spans the two chains, called Proof of Transfer. This enables Stacks to leverage Bitcoin’s security and enables Stacks apps to use Bitcoin’s state, despite being a separate blockchain.
Instead of starting from scratch, Stacks extends Bitcoin with new functionality. Bitcoin is Stacks’ secure and robust base layer where all transactions are settled, and Stacks adds complex apps and smart contracts. Stacks apps can interact with Bitcoin state, so you can have an app that uses Bitcoin as its currency.