Blockchain technology, originally developed as the underlying technology behind the cryptocurrency Bitcoin, has evolved beyond just a digital currency. The technology has found multiple use cases in different industries, with fintech being one of the major beneficiaries. Blockchain technology is disrupting the financial industry by enabling decentralized finance (DeFi). DeFi is a decentralized financial system built on a blockchain, designed to provide financial services to everyone without intermediaries like banks, governments, or other financial institutions. In this article, we will explore blockchain, fintech, and DeFi in-depth. We will discuss the value and impact of the technology, the architecture of DeFi, provide a sample code of DeFi on Ethereum blockchain, the problems DeFi solves, and how to bring DeFi to the mass market.