What is DeFi?
DeFi (or Decentralized Finance) is a financial system in which all financial institutions and financial instruments are operated in a decentralized way. In other words, DeFi leverages the decentralization feature of Blockchain to create what is called open finance. In an open financial system, everyone has access to financial services without the need for permission of any individual or organization.

DeFi is attached with the key property, non-custodial.
DeFi also has a complete structure of financial institutions and instruments like CeFi:
It also has all of the financial activities in CeFi from saving, lending, and borrowing to bills and debt payments,...
The difference is that all of these activities are not conducted by a central organization but automatically by the smart contracts of Blockchain.

DeFi represents a groundbreaking shift in finance, democratizing access to services by removing intermediaries. Its reliance on smart contracts ensures transparency and efficiency, but it also raises questions about security and regulatory oversight in this rapidly evolving landscape.