Bitcoin - Mining
In the image, Bob's network is depicted, and when he creates a purchase request for Lisa, it is not sent to Lisa alone. Instead, the request is broadcast to the entire network Bob is connected to.
There are some nodes in the diagram that are marked as miners. These are machines that run a piece of software that mines the bitcoin message. I will explain what this means next.
Mining Process
Every miner in the network receives multiple messages from multiple vendors at any given time. What he does is combine these messages into a single block.
Once a block of messages has been formed, the miner hashes the block using the hashing function. Now, as you probably know, if a third party modifies the block's contents, its hash becomes invalid. A message in the block is timestamped in such a way that no one can change its chronological order without affecting the block's hash value. As a result, the messages in the block are perfectly secure. This fact is used to secure all transactions in the network.