ChainLink is a decentralized oracle (software built on top of a blockchain), which provides external data to Ethereum smart contracts. ChainLink is an open source project, started at September 2017 by a company SmartContract. This company was already creating secure oracles for Bitcoin and Ethereum for over 3 years, but in 2017 decided to move towards their own middleware ChainLink. At the moment ChainLink is maintained by 3 developers.
The main purpose for creating ChainLink was to provide nnectivity between blockchains and external software such as bank payments, retail payments (Visa, Paypal), market data (NYSE, Bloomberg) and backend systems (SalesForce, SAP). This allows to integrate blockchain with off-chain sales and local payments, to turn them into smart contracts in exchange for LINK tockens. LINK token is an EIP20 Ethereum token with some additional functions. ICO for LINK tokens was completed on September 2017 raising total of $32.00 M.
Currently ChainLink only works with Ropsten, a test Ethereum network.