CEX (centralized exchange) — Bybit, Binance, OKX. Account, KYC, order book, withdrawal to an address.
DEX (decentralized exchange) — Uniswap, Jupiter, STON.fi. Wallet, signature, pool.
When CEX
Fiat, large pairs, a familiar order. You are in the custodial zone while coins sit on the exchange. See an exchange wallet.
When DEX
A token your exchange does not list. A chain you already funded with gas and a stablecoin. Not wanting the stack to sit on a firm's books.
How they join
Almost everyone's path is: fiat → CEX → withdraw to a chain → DEX. Skipping “withdraw on the right network” is the classic lost USDT.