A centralised exchange, sometimes known as a CEX, is an exchange that is owned and operated by a business. This business creates and controls various investment products and when you invest in one of these products you are handing control of your funds to this business. A decentralised exchange, sometimes known as a DEX, is an exchange that is not controlled by any single entity. The exchange will still have been created by a group of people, but it will normally be decentralised in its adminstration and control mechanism.