Chainlink is a protocol built on the Ethereum blockchain that uses a distributed system of oracles to connect, or link, off-chain data sources to blockchains, hence the name Chainlink.

The potential off-chain data sources include virtually any API call to any third party data source. Use case examples include stock exchange data, weather data, political and sporting event data, and academic certification data to name a few.

The Chainlink's native ERC-20 token, LINK, is used to incentivize the passage of truthful data by oracles from off-chain sources. Oracles must stake LINK, and risk losing LINK tokens if they provide inaccurate data.

In the simplest possible terms, Chainlink links off-chain APIs to blockchains.