Lets understand what does it means, on Decentralized exchange, we (traders) won’t have to rely on third party services. Traders can directly do the trade between each other (peer to peer).  The most amazing thing about “DEX” is You are not required to trust the security or honesty of the exchange because the funds are held by you in your personal wallet and not by a third party. For login on “DEX” you just need a assets wallet. Another benefit of “DEX” is that users are not required to disclose their personal details to anyone except if the exchange method involves bank transfers, in which case your identity is revealed only to the person that is selling or buying from you. “DEX” reduces the chances of hacking & server down problem. 
Even DEX have some limitation like while trading user must be online, features like margin trading, lending and stop loss are currently not available. 
Following are names of “DEX”
Bitsquar, NXT, Switcheo (Based on NEO NEP5), CounterParty DEX and some in development also.
Note:- This is my opinion about “DEX”. This post is not sponsored by ANY “DEX” for promotion. Be wise while trading for cryptocurrency