Chainlink (LINK) was created in 2017 by Sergey Nazarov and Steve Ellis, who co-authored a white paper introducing the Chainlink protocol and network with Cornell University professor Ari Juels the same year. Chainlink acts as a "bridge" between a blockchain and off-chain environments. The network is used by smart contracts and was formally launched in 2019. Chainlink's LINK token is an ERC677 token, an extension of ERC20. Tokens act as data payloads, feeding the required data from off-chain sources to smart contracts, which then act accordingly in response to the data provided by the token.
Chainlink oracles act as a bridge between two environments, on-chain and off-chain.
With Chainlink oracles, blockchains like Ethereum, Bitcoin, Hyperledger, and others can connect to off-chain resources such as market data, bank payments, retail payments, backend systems, events data, other blockchains, and more via web APIs.