Decentralized Exchanges (DEX) are crypto currency exchange platforms that facilitate exchanging between clients where each client has control over his own crypto assets.
How Do they operate?
Since they are decentralized and do not store crypto assets for clients they just do the exchanging and swapping only. Lets say you have Bitcoins they you go on changenow and you want Ethereum instead, you just send Bitcoins to their wallet and they sent you the equivalent amount of Ethereum and the deal is done.
You might have some Ethereum and you want an ERC-20 token you just swap and you re charged transaction costs and swapping costs which are sometimes high and if its an ERC-20 you are charged in ETH.
Examples of DEX
Its fast as transactions are done instantly and its safe as well since everything is decentralized they are no cases of hacking or losing your coins or tokens.