People would need to keep their computers on for this to work right? What if you need to access your data and they turned their computer off? Great idea though, I'm excited to see how this turns out.
The files are broken into pieces called shards. Each Shard is written to 3 or more computers, similar to how a RAID drive does. This creates a fault tolerance to the network. A heartbeat is sent to each node and if the node is found to be offline then the shard that were on that node are replicated elsewhere.
So a file that contains 5 shards could be saved to 15 different computers.