“Dapp” stands for “decentralized application.”
Decentralized means it’s not controlled by one entity. Most applications on the internet, including the most well known, are centralized applications controlled by one company. Think Facebook, Twitter, Google, Yahoo and many other applications large and small. Currently, these are ‘not’ dapps.
More specifically Dapps are programs, tools, or applications that run on the decentralized Ethereum blockchain.
A blockchain is a digitized, decentralized, public ledger of data, made famous as the foundation of all decentralized cryptocurrency transactions.
Blockchains can support many more applications than money.
The Ethereum network is a popular blockchain. As such, Ethereum is a decentralized technology uncontrolled by any single authority.
To further elaborate, regular apps have all their data controlled by their own company servers with one single authority. They may require a user login that collects personal information, such as name, birth date, address, user history, etc.
Alternatively, Dapps work off a blockchain. Instead of personal information for logging in, they require a private address which can be a random string of characters that holds no personal information.
We are in the early days of Dapp development and you will be hearing more about this term as conventional applications shift from the universe of centralized control to blockchain decentralized application.