This chapter introduces Decentralized Finance (DeFi), a rapidly growing area that enables financial transactions such as lending, borrowing, and trading to occur without traditional intermediaries. You will learn the foundational components of DeFi by exploring token standards, such as ERC20 and ERC721, stablecoins and their mechanisms for maintaining price stability, and wrapped tokens that facilitate cross-chain interoperability. The chapter explains how DeFi protocols are structured and how they function, focusing on core areas including decentralized exchanges, lending protocols, derivative markets, and yield farming. You will gain an understanding of key financial concepts adapted to blockchain systems, such as leverage, hedging, and impermanent loss. Particular attention is given to algorithmic innovations like automated market makers, flash loans, and synthetic asset creation. You will also study how oracles feed real-world data into smart contracts and how decentralized governance structures such as DAOs shape protocol evolution. Throughout the chapter, we emphasize the risks, incentives, and architectural trade-offs involved in designing and interacting with DeFi systems.