Asslam-o-alaikum friends and dear sir @yousafharoonkhan. I hope you all are well.In this post I am going to perform my home work task which is,define decentralized exchange and centralized exchange.
A decentralized exchange is an exchange in which there is no third person between you and the buyer, which we also call the middle man. It does not involve any action other than these two.
Diagram of Decentralized exchange:

Image source:Oppo A37 mobile
For example:
You have 5 bitcoin dollars that you want to avoid or exchange. You go to a platform that is a decentralized exchange platform. For example uniswap, you go there to sell them and there is someone who wants to buy 5 bitcoin dollars and contact them or they contact you. And your matching is done and your deal is done. You don't need a third person. You as a seller and the buyer did your exchange work directly.

Real life example of decentralized exchange:

source
If you want to buy something from a bazaar. Simply, you go to bazaar and buy it. In this you have no need a third person Like you say you need the item and he or she makes a deal between you and the shopkeeper to meet your and the shopkeeper's demand.
centralized exchange:
A centralized exchange is an exchange in which you and the buyer need a third person to deal. There is another person besides the seller and the buyer who is called the third person.
Diagram of centralized exchange:

Image source:oppo A37 mobile

For example:
We use the same decentralized example in this as well. In this you fulfill your deal by negotiating end to end but in this you need a third person. For example, binance is a third person between you and the buyer who helps to complete the deal between you and the buyer and also charges a fee.
Real life example of centralized exchange:

source
If you want to buy a flat or a house, you first have to talk to a property dealer and he will tell you about a house that meets the requirements and you like. Even if So the dealer will complete the deal between you and the seller and charge you for it. He will act as a third person between you and the homeowner.
I hope you enjoyed my homework task post.
Special thanks:
@yousafharoonkhan