A decentralized application is a purpose-specific decentralized system. For instance, while Ethereum is a decentralized network because anyone can join and the nodes are all peer-to-peer, a decentralized application will run on top of the network to provide a specific service or set of services to users. To some extent, the distinction is moot—you could see Ethereum as a distributed application that provides smart contract services and native token transfers. In any case, the key distinction is about power.