A DApp is a kind of application whose backend runs on a decentralized peer-to-peer network, and its source code is open source.No single node in the network has complete control of the DApp. Remember that, when we say that an application is decentralized we mean technically it's decentralized but the governance can be distributed, decentralized, or centralized.
The major advantages of DApps are that they don't have a single point of failure, and prevent censorship. DApps do have some disadvantages: it's difficult to fix bugs or add features once deployed as everyone in the network has to update their node software, and it's very complicated to couple different DApps together as they are very difficult to build compared to centralized applications and involve very complex protocols.
To be able to use a DApp, you first need the DApp's node server running so that you can connect to the peer-to-peer network. Then, you need a client respective to the DApp that connects to the node server and exposes a UI or command line interface to use the DApp.
Currently, DApps are not yet as mature as centralized applications in terms of performance and scalability.There is still a lot of research and development on these topics such as performance, scalability, users identity, privacy, communication between DApps, data redundancy,and so on.A use case may fit into a DApp, but whether the use case can be made production-ready with the currently available technology can be a challenge. Popular examples of decentralized applications are Torrent, Bitcoin, Ethereum, Quorum, and so on.
A DApp can be public or permissioned. Public DApps are those which anyone can be part of, in other words, they are permissionless, whereas permissioned DApps are those which are not open for everyone to join, so you will need permission to join. A permissioned DApp is called a consortium DApp when the participants of the DApp are enterprises and/or government entities. Similarly, when the participants of a permissioned DApp are only enterprises, then we can call it an enterprise DApp. In this book we will learn everything about permissioned DApps.
Note
As you just got a basic introduction to what decentralized applications are, you must be wondering what the difference between decentralized and distributed applications is. Well, an application is said to be distributed when it's spread across multiple servers. Decentralized applications are by default distributed, whereas centralized applications may or may not be distributed. Centralized applications are usually distributed across multiple servers to prevent downtime, and also to handle huge data and traffic.