Both Swarm and Whisper are complementary technologies contributing to the vision of Ethereum as a "world computer". When imagining Ethereum as a metaphor for a shared computer, it should be noted that computation alone is not enough. For a computer to be fully useful, it also needs storage to "remember" things and bandwidth to "communicate" them. This could be summarised as such:

Contracts : decentralized logic

: decentralized logic Swarm : decentralized storage

: decentralized storage Whisper: decentralized messaging

More specifically, Swarm is being designed as an accounting protocol that benefits from the automatic execution of so-called "smart contracts" running on the Ethereum Virtual Machine (EVM). This accounting protocol is independent of the physical storage mechanism. That is, it is not intrinsically tied to a specific storage system. It could be IPFS, BitTorrent, or some future technology not yet invented.

It is not entirely clear where Swarm and Whisper fit into the development roadmap and when or if they will be delivered by the same developers that developed the EVM, however it is part of the vision of a fully decentralized web.

UPDATE: swarm landing page via swarm public gateway 5 as pf 12 May 2016