A distributed system is the distribution of computational power across many different places instead of just one centralised processing location. For example, if we are facebook, we service the entire world.
So we may have data centers at all major continent hubs to ensure fast processing. So an application (e.g facebook) can be centralised but distributed.
Decentralised is different; it means that there is no central location, all the ‘nodes’ that make up the decentralised system are not instructing any other node what to do.