https://github.com/ethereum/go-ethereum/wiki/Whisper-Overview
Whisper Overview
Whisper is a pure identity-based messaging system. Whisper provides a simple low-level API without being based upon or influenced by the low-level hardware attributes and characteristics. Peer-to-peer communication between the nodes of Whisper network uses the underlying ÐΞVp2p Wire Protocol. Whisper was not designed to provide a connection-oriented system, nor for simply delivering data between a pair of particular network endpoints. However, this might be necessary in some very specific cases (e.g. delivering the expired messages in case they were missed), and Whisper protocol will accommodate for that. Whisper is designed for easy and efficient broadcasting, and also for low-level asynchronous communications. It is designed to be a building block in next generation of unstoppable ÐApps. It was designed to provide resilience and privacy at considerable expense. At its most secure mode of operation, Whisper can theoretically deliver 100% darkness. Whisper should also allow the users to configure the level of privacy (how much information it leaks concerning the ÐApp content and ultimately, user activities) as a trade-off for performance.
Basically, all Whisper messages are supposed to be sent to every Whisper node. In order to prevent a DDoS attack, proof-of-work (PoW) algorithm is used. Messages will be processed (and forwarded further) only if their PoW exceeds a certain threshold, otherwise they will be dropped.