How does IPFS relate with Ethereum?

From @Piruzzolo on Sun Apr 03 2016 13:05:45 GMT+0000 (UTC)

Could you give an overview of how these two technologies could work together (if possible at all)?
Can Ethereum lay upon IPFS?

Copied from original issue:

You can store IPFS Hash in smart contract. Suppose you want display any important file/document. You can leverage immutability of Ethereum Smart contract and store ipfs hash of that file.