In Holochain’s own words: "Holochain provides a data structure that can be used to build decentralized apps. You have your own chain, which you can add data to, including financial transactions. The chains can merge, split, and interact in complex ways. The data is stored in a decentralized way (like Bittorrent). The data has a hash, which is a mathematical fingerprint that corresponds to the data. If someone tampers with the data, the mismatch between the data and the hash will be noticed, and the data rejected as invalid. Digital signatures guarantee the authorship of the data. It’s Bittorrent plus git plus digital signatures."
Blockchains store the database in a decentralised way but the data itself is not really distributed as it is repeated in full in every node. By comparison, Holochain fractionalises data. The network no longer keeps n-thousand copies in every node but only a user’s own data and sufficient copies of other data to ensure and confirm network veracity and consistency. Then every user acts as a node, and nodes can be small and lightweight enough to fit on a mobile phone: