Processing - mining
Mining is a distributed consensus system that is used to confirm pending transactions by including them in the block chain. It enforces a chronological order in the block chain, protects the neutrality of the network, and allows different computers to agree on the state of the system.
To be confirmed, transactions must be packed in a block that fits very strict cryptographic rules that will be verified by the network. These rules prevent previous blocks from being modified because doing so would invalidate all the subsequent blocks.
Mining also creates the equivalent of a competitive lottery that prevents any individual from easily adding new blocks consecutively to the block chain. In this way, no group or individuals can control what is included in the block chain or replace parts of the block chain to roll back their own spends.