Decentralized Applications (DApps) are the lifeblood of the Ethereum ecosystem. DApps are typically composed of on-chain smart contracts that process and store data paired with a front-end application for display and interaction.
DApps can take many forms including games, finance, social, bridges, voting apps - basically any application which can be configured to run in a distributed, decentralized network.
Developers choose POA network (and the Sokol testnet and xDai Chain) to run their DApps for two primary reasons. Transactions process quickly and fees are minimal. With the TokenBridge architecture, projects can also create DApps that utilize POA-based networks for the "heavy lifting" and then leverage bridges to move assets procured on a POA-based network to the Ethereum Mainnet.
A short list of DApps running on POA Mainnet (this is not an exhaustive list, only apps that have been submitted) is available at https://www.stateofthedapps.com/platforms/poa