Bitcoin is a peer-to-peer, or node-to-node electronic cash system. Every Bitcoin wallet acts as a node. Among them, full nodes store the entire ledger. As of October 2017, the Bitcoin network has around 9300 full nodes running, responsible for broadcasting and validating transactions.
When a transaction is initiated, all nodes will relay the information to the entire network. After validation, nodes will process the transaction into the blockchain. USA, Germany and France have the greatest number of Bitcoin full nodes.