EDIT: Well there wasn't much information in the OP or the official site but a quick read of the whitepaper suggests that data won't actually be stored in the blockchain but rather on other people's hard drives. How will I know that my data will stay backed up unless every node in the network has a copy of it - i.e. a blockchain? Doesn't this mean that a few computers in the network can be switched off, break down, or their owner(s) might decide to reformat their hard drives and my novel can disappear forever while the network - oblivious to all of this, continues chugging along?
Yes we won't need a blockchain to store data but we will use DriveShare nodes around the world for that. Everyone can share their extra hd space.
Files will become unavailable when the uploader stops paying for them to be stored at the chosen redundancy. By default the redundancy will be set to 3x.
We will have algorithms and automatic checks on the network (heartbeats) that will allow to keep track of which file chunks are unreachable because a node has gone down or has been turned off.
In that case a replica of the chunk will be made and stored on an another available node to maintain the required redundancy.
As fox19891989 mentioned have a read through our FAQ on StorjTalk.org to get a bit more familiar with the project and feel free to join our forum where you can also ask any question you may have.