ERC-20 walkthrough - https://ethereum.org/en/developers/do...

EIP-20: Token Standard (technical spec) - https://eips.ethereum.org/EIPS/eip-20

Remix, Solidity IDE - https://remix.ethereum.org/

ERC-20 is a technical standard for tokens on Ethereum blockchain; it is used for all smart contracts on the Ethereum blockchain for token implementation and provides a list of rules that all Ethereum-based tokens must follow.

ERC-20 is similar, in some respects, to bitcoin, Litecoin, and any other cryptocurrency; ERC-20 tokens are blockchain-based assets that have value and can be sent and received. The primary difference is that instead of running on their own blockchain, ERC-20 tokens are issued on the Ethereum network.