What DeFi Is

1.1 A bank made of code

Mint wanted to earn interest on some stablecoins. A bank would take days to open an account, ask for documents, and pay very little. A DeFi lending protocol took two minutes: connect a wallet, approve, deposit. Interest started accruing immediately.

"Who's paying me?" Mint asked. "There's no bank."

"Borrowers," said Hash. "Through a smart contract that matches them with lenders automatically, sets interest rates by formula, and liquidates borrowers who don't keep enough collateral. No staff, no branch, no opening hours. That's DeFi. It's powerful, and it fails in ways banks don't. This module is about both."

1.2 What DeFi is

Decentralised finance (DeFi) is the name for financial services built on smart contracts (Module 1, Chapter 4): trading, lending, borrowing, stablecoins, derivatives, staking, and more, running on public blockchains without a central company holding users' funds.

You've already met many DeFi building blocks in this series:

Building block	Introduced in
Decentralised exchanges and AMMs	Module 1, Chapter 8
Stablecoins	Module 1, Chapter 5
Token approvals and signatures	Module 2, Chapters 7 and 8
Gas, MEV, and oracles	Module 3, Chapters 2, 11, and 12
Evaluating DeFi protocols	Module 5, Chapter 10
On-chain DeFi data	Module 6, Chapter 9
Decentralised perps and options vaults	Modules 7 and 8

This module brings them together and goes deeper.

1.3 The core properties

Property	What it means	Why it matters
Non-custodial	You keep control of your assets until you deposit them into a smart contract, and often can withdraw any time	No company can freeze or misuse your funds, though contracts can fail
Permissionless	Anyone with a wallet can use it, usually without accounts or approval	Open to anyone, including people excluded from traditional finance
Transparent	All transactions and balances are visible on-chain	Risks and activity can be checked by anyone (Module 6)
Composable	Protocols can plug into each other like building blocks	Rapid innovation, and chains of dependency
Automated	Rules are enforced by code, without human discretion	Predictable, but unforgiving of bugs and edge cases

1.4 Money legos

Composability is often described as "money legos." Because DeFi protocols are open smart contracts, any protocol can use any other.

Here's a simple stack:

1. Mint deposits ETH into a liquid staking protocol and receives a token representing staked ETH (Chapter 8).
2. Mint deposits that token into a lending protocol as collateral (Chapter 5).
3. Mint borrows stablecoins against it (Chapter 7).
4. Mint deposits those stablecoins into a liquidity pool on a DEX (Chapter 3).

Four protocols, each building on the last, all in minutes, with no applications or approvals. That's remarkable. It's also a chain where any weak link can break the whole thing (Chapter 12).

"So it's like building with blocks where every block can fail independently," said Node.

"And where some blocks hold up others," said Hash. "Composability multiplies both the possibilities and the risks."

Chapter 1 · Money legos

ETH is staked into a liquid staking token, which is supplied as collateral to a lending market, against which USDC is borrowed and deposited into a DEX liquidity pool. Each layer adds contract, oracle, and liquidation risk from every layer beneath it.

1.5 DeFi compared with traditional finance and CeFi

	Traditional finance	Centralised crypto finance (CeFi)	DeFi
Who holds funds	Banks and brokers	Crypto companies	Smart contracts
Access	Accounts, identity checks	Accounts, usually KYC	A wallet
Opening hours	Business hours for many services	24/7	24/7
Transparency	Limited, periodic reports	Limited, sometimes proof of reserves	Full on-chain visibility
Recourse if something goes wrong	Regulation, deposit insurance, courts	Varies; often limited	Usually none
Main risks	Institutional failure, though often insured	Company fraud or failure (Module 1, Chapter 7)	Code bugs, oracle and economic failures, user error

The 2022 collapses of several crypto lending companies, which took customer deposits and made risky loans behind closed doors, were CeFi failures. Many DeFi protocols operating transparently on-chain continued working through that period, with liquidations executing automatically. That episode made DeFi's transparency argument concrete. But DeFi has had plenty of failures of its own, through hacks, bugs, and broken economic designs.

1.6 Where DeFi yields come from

Before using any DeFi product, ask where the yield comes from (Module 2, Chapter 11, and Module 5, Chapter 10).

Source	Example	Sustainability
Borrowers paying interest	Lending protocols	Depends on borrowing demand
Traders paying fees	DEX liquidity provision	Depends on trading volume, offset by impermanent loss (Chapter 4)
Network rewards	Staking (Chapter 8)	Tied to network issuance and fees
Funding and basis	Delta-neutral strategies (Module 7, Chapter 12)	Depends on market conditions
Token incentives	Liquidity mining (Chapter 10)	Temporary; paid through dilution
Points and expected airdrops	New protocols	Speculative

1.7 The DeFi risk map

Risk	Example	Chapter
Smart contract	A bug lets an attacker drain funds	13
Oracle	Manipulated prices trigger bad liquidations (Module 3, Chapter 12)	13
Economic design	A stablecoin's mechanism fails under stress	7 and 12
Liquidation	Collateral falls in value and your position is closed	6
Impermanent loss	Price changes reduce liquidity provider value	4
Composability and contagion	A failure in one protocol spreads to others	12
Governance and admin keys	Parameters changed or contracts upgraded against users' interests	13
User error	Wrong approvals, phishing, or sending to wrong networks (Module 2)	13

1.8 Try it yourself

Open a DeFi analytics dashboard that lists protocols by category. Note the largest protocols in DEXs, lending, liquid staking, and stablecoins, and their total value locked. Then pick one lending protocol and find the current supply and borrow rates for USDC. Where would that supply rate come from?

Key takeaways from this chapter

1. DeFi provides financial services through smart contracts on public blockchains, without central custodians.
2. Its core properties are non-custodial, permissionless, transparent, composable, and automated.
3. Composability lets protocols stack like building blocks, multiplying both innovation and risk.
4. DeFi differs from traditional finance and CeFi in custody, access, transparency, recourse, and risks.
5. Always trace where a yield comes from: borrowers, traders, network rewards, funding, incentives, or speculation.
6. DeFi risks include smart contracts, oracles, economic design, liquidations, impermanent loss, contagion, governance, and user error.