Ethereum Request for Comment (ERC) standards are explained as part of the Ethereum Improvement Proposal (EIP) process, which is where they are written, numbered and debated. This page gives a short definition and explains why the two labels are used side by side.
An ERC is a standard for how applications on Ethereum should behave, rather than a change to the core protocol. It sets out common function names and rules so that wallets, exchanges and smart contracts built by different teams can work together. The best-known example is ERC-20, the template for fungible tokens, which lets any wallet display and send any compliant token. Others include ERC-721 for non-fungible tokens, ERC-1155 for contracts holding many token types, and ERC-4337 for account abstraction.
The name comes from the "Request for Comments" documents used to agree internet standards. In Ethereum, every ERC begins life as an EIP in the "ERC" category, which is why the same document can be called ERC-20 or EIP-20. In everyday use, "ERC" is kept for application and token standards, while "EIP" is often used for changes to the network itself.