Chaincode is a program (or programs) that runs on top of the Hyperledger Fabric blockchain to implement the business logic of how applications interact with the ledger. It is currently written in Go, Node.js, and eventually will be written in other programming languages, such as Java, that implement a prescribed interface. When a transaction is invoked, it triggers the chaincode that decides what state change should be applied to the ledger. Chaincode is typically considered  a smart contract. A state created by chaincode is restricted in scope to that chaincode and can't be accessed directly by another chaincode.