Description from cryptocompare.com
Sia is an actively developed decentralized storage platform. Users all over the world contribute disk storage from their computers to form a decentralized network.
Anybody with siacoins can rent storage from hosts on Sia. This is accomplish via "smart" storage contracts stored on the Sia blockchain. The smart contract provides a payment to the host only after the host has kept the file for a given amount of time. If the host loses the file, the host does not get paid.
The distrubuted nature of the Sia network enables many optimizations in latency, throughput, reliability, and security. The decentralized nature of the Sia network enables anyone with storage to get paid, lowering the barrier to entry and reducing the overall price of cloud storage.
The Sia cryptocurrency is live! You can rent storage using siacoins and hosts providing storage to the network receive compensation in the form of siacoins. The storage platform itself is still in beta, and only uploads that are 500mb or less in size are supported by the wallet.