DApp is an abbreviation for Decentralized Application. DApps connect users and providers directly.
Swarm City is listed on the State of DApps site
The Ethereum Stack Exchange explains it like this...
A DApp runs backend code on a global decentralized peer-to-peer network. Contrast this with an app where the backend code is running on centralized servers. A DApp can have frontend code and user interfaces written in any language (just like an app) that can make calls to it’s backend since Ethereum contracts are code that run on the global Ethereum decentralized network.
A traditional app = "frontend + servers"
A DApp = "frontend + contracts"
Read this Coindesk article for a more in depth look.