With regards to Ethereum, decentralized applications, or Dapps, are interfaces which interact with the system through the execution of smart contracts.
On the surface, Dapps are similar to standard mobile and web applications, with the exception that they interact with a blockchain in several ways.
Among these ways are storing data on the Ethereum blockchain so as to be immutable, requiring Ether (ETH) for usage of Dapps, and others.
