On the exchanges you are used to using you send your coins there and the exchange has them. You can buy or sell coins at the exchange and you are limited to what the exchange pairs together for buying and selling. Also, in many cases the exchange will harvest the staking and also if an exchange goes down you may or may not get your coins.

A de-centralized exchange is very different than this. This is a peer to peer network of users. The de-centralized exchange is actually a software running on your computer that can communicate with your local wallet(s) and it can also communicate with all the other people and their wallets that are connected with the software. In this situation your coins stay on your computer and you get all the staking rewards. Also, this gives you the option to pair your sell order with any other coin. For example you can put ROI up for sale and exchange it for any coin that is listed at the de-centralized exchange.