bitcoin (P2P electronic cash system) Bitcoin is a free open source peer-to-peer electronic cash system that is completely decentralized, without the need for a central server or trusted parties. Users hold the crypto keys to their own money and transact directly with each other, with the help of a P2P network to check for double-spending. This Bitcoin client builds the backbone of the bitcoin network. It downloads, verifies and distributes the blockchain to other peers in the network. It uses lots of diskspace and bandwith. If you just want to send and receive Bitcoins you should look at a light client like Electrum. 0.10.0 is a major version upgrade which makes use of headers-first synchronization and parallel block download, the block files and databases are not backwards-compatible with older versions of Bitcoin Core or other software. Please make sure you read the Release notes first before upgrading: https://bitcoin.org/en/release/v0.10.0 qrencode is an optional but strongly recommended dependency. UPNP is disabled, it requires the optional dependency miniupnpc.