Chainlink connects smart contracts with external data using its decentralized oracle network. Chainlink API requests are handled 1:1 by an oracle.
The Basic Request Model describes the on-chain architecture of requesting data from a single oracle source.
To learn how to make a GET request using a single oracle, see Make a GET Request.
For a more robust and trustworthy answer, you can aggregate data from many oracles. With on-chain aggregation, data is aggregated from a decentralized network of independent oracle nodes. This architecture is applied to Chainlink Data Feeds, which can aggregate data such as asset price data.
The Decentralized Data Model describes how data is aggregated, and how consumer contracts can retrieve this data.
To learn how to consume the price of ETH in your smart contract, see Get the Latest Price.