Summary: A dApp is a decentralized app. It is a blockchain-based service and program, encompassing everything from the base code to the user interface. A dApp can allow a huge variety of new abilities by making use of blockchain smart contracts.
What is a dApp?
The simple answer is that dApps are decentralized apps. And what are decentralized apps? Well, that’s more complicated.
On the most basic level, a dApp is any application that runs on a blockchain, and which thus runs in a distributed manner. So, for instance, Bitcoin is a dApp, one which runs on the Bitcoin blockchain. Ethereum, a blockchain that allows anyone to build blockchain-based services is itself a dApp -- even though its own core function is to allow the the creation and hosting of further dApps. So Ethereum, and similar projects like EOS and NEO, are dApps for making and hosting dApps.
What this means is that the term dApp encompasses a very wide array of types of software, and it requires a broader definition of phrases like “on the blockchain.” What proportion of a service needs to be administered via a blockchain in order for the service itself to be considered a dApp?
In principle, for a dApp to be a dApp, it must host any sensitive records and actionable pieces of information in a distributed ledger, AKA, a blockchain. This means that portions of the service can be hosted and delivered like a regular online service, just so long as the most crucially important aspects of the service have been decentralized and protected via blockchain tech.So, a decentralized cloud computing environment could have all the actual computing occur on regular cloud computing servers and use the blockchain only to keep make them available properly organized, and handled payment for services rendered.
Remember that storage space is at a premium on blockchains, so the goal is to do as little of the actual work on the blockchain as possible, while still decentralizing enough of the process to allow all new abilities.
dApps can run on mobile devices, or they can be entirely web-based. They can be large-scale organizational structures or tiny, highly focused solutions. They can change the world through decentralization, or enable the continuing evolution of the blockchain itself.
dApps are the platform in which smart contracts are executed -- which means they can take almost any digital act we can imagine.