dApp is an abbreviation for Decentralized Application.
These are applications that do not rely on a central point or server to run. Instead, the resources that application needs are provided by people providing other people the information, services or computing power that they seek thru a decentralized backbone which is usually blockchain based.
In the example of Ethereum or EOS, applications deployed on the blockchain are run by a distributed network of computers which means that no single entity can control or censor the application and its users.
This grants true inclusivity for everyone as anyone with access to the internet and the blockchain can make use of the dApp. Furthermore dApps are more transparent than traditional applications since data and actions performed can be audited by anyone.
dApps are usually open-source to allow the public to audit the code of the application and spot potential errors or bugs.
A good example of a dApp is Cryptokitties a game on the blockchain where you can take care of, breed and trade digital cats. Each cat is represented by a bespoke or unique token with each cat’s unique characteristic in the token. These tokens can then be sent to your wallet for storage.