Founded in 2017 by SmartContract, Chainlink is a decentralized network of nodes that provide data and information from off-blockchain sources to onblockchain smart contracts via oracles. An oracle is a software acting as an intermediary (the “middle-man”) between smart contracts and the off-blockchain data providers.
Connecting outside information sources to smart contracts is vital to the use cases of smart contracts and blockchains. The challenge of getting off-chain information (e.g. financial data) on-chain in a reliable, trust-minimized manner is generally known as the "oracle problem" and was largely unsolved until the formation of Chainlink.
Chainlink’s token (often also called Link) is an Ethereum-based ERC-20 token used to pay for the services of Chainlink nodes providing off-blockchain data to smart contracts.
Chainlink allows smart contract to connect to the outside world providing reliable, tamper-proof inputs and outputs for complex smart contracts on any blockchain.