What does Dex do?

So, can you tell me, in simple terms, exactly what does a decentralized exchange, or Dex, do? I've heard it mentioned a lot in the world of cryptocurrency but I'm still a bit hazy on the specifics. I understand it's a type of platform that allows for the trading of digital assets, but how does it differ from a traditional centralized exchange, and what kind of benefits does it offer users?