Glossary
CEX
Centralized exchange: a custodial trading venue (Binance, Coinbase, Kraken) where users deposit funds and trade via the platform's internal order book.
A CEX runs a private database of customer balances, matches orders internally, and only touches the blockchain on deposits and withdrawals. That is why CEX trading is fast and cheap — the actual trading is just rows in a database.
The cost is custody: while your funds are on the exchange, the exchange controls them. FTX, Mt. Gox, and QuadrigaCX all proved that exchange failure can wipe out customer balances, which is the source of the maxim "not your keys, not your coins".