A quick tour of the ecosystem
Because Ethereum can run arbitrary programs, a whole ecosystem of applications has grown on top of it, and it helps to know the shape of it even without going deep into any one piece. Fungible tokens (built to a standard called ERC-20) are contracts that behave like programmable currencies, every token you've heard of that isn't ETH itself, like a stablecoin or a governance token, is really just a smart contract tracking balances. NFTs (ERC-721 and related standards) use that same contract pattern but track unique, non-interchangeable items instead of interchangeable balances, which is why each NFT can point to distinct data or ownership rather than being identical to every other unit.
Decentralized exchanges are contracts that let people swap one token for another without a company running an order book, often using pooled funds and a pricing formula instead of matching buyers to sellers directly. Lending protocols are contracts that let people deposit assets to earn interest or borrow against collateral they've locked up, all enforced automatically by code rather than a bank's loan officer. None of these are separate technologies bolted onto Ethereum, they're all just smart contracts, the same building block from earlier lessons, arranged to do different jobs.