Decentralized Exchanges (DEX) are aiming to operate trading without the authority of any central governing figure. Which means that the trading platform runs only by the blockchain which dont hold any kind of asset or information but allows transactions between users to take place.
Centralized Exchanges (CEX) are just the opposite of DEX and a whole lot more.
Read more here: https://kucoinblog.com/are-decentralized-exchanges-the-future-sk-st