Decentralized applications (dApps) are applications or programs that exist and run on a blockchain or P2P network of computers instead of a single computer, and are not under the jurisdiction and control of a single authority.
A standard web application, such as Uber or Twitter, runs on a computer system that is owned and operated by an organization, giving it full authority over the application and its operation.
BitTorrent, Tor and Popcorn Time are examples of applications that are part of a P2P network where there are multiple participants on all sides, some consuming the content, some feeding it and some performing both functions simultaneously.
In the context of cryptocurrencies, dApps exist and operate on a blockchain in a public, open source, decentralized environment and are free from control and interference from any single authority.
A developer can create a Twitter-like dApp and place it on a blockchain, where any user can tweet messages. Once posted, no one (including the creators of the app) can delete the tweets. Editing may be possible by the sender, but the original tweet would be retained forever. For example, a decentralized version of YouTube has been developed, it is called Dtube.
The website dappradar.com list a lot of dapp with the volume associated with each of them.
