The working paper (pdf) describes it in this way:
Filecoin is a distributed electronic currency similar to Bitcoin. Unlike Bitcoin’s computation-only proof-of-work, Filecoin’s proof-of-work function includes a proof-of-retrievability component, which requires nodes to prove they store a particular file. The Filecoin network forms an entirely distributed file storage system, whose nodes are incentivized to store as much of the entire network’s data as they can. The currency is awarded for storing files, and is transferred in transactions, as in Bitcoin. Files are added to the network by spending currency. This produces strong monetary incentives for individuals to join and work for the network. In the course of ordinary operation of the Filecoin network, nodes contribute useful work in the form of storage and distribution of valuable data.
The mother site is here. File under…arbitrage.
For the pointer I thank J.