Anyone can become a Bitcoin miner by running software on specialized hardware. Mining software listens to transactions broadcast over the P2P network and performs the appropriate tasks to process and confirm those transactions. Bitcoin miners earn transaction fees paid by users to speed up transaction processing, as well as additional Bitcoins issued according to a fixed formula.
In order for a new transaction to be confirmed, it must be included in a block with a mathematical proof of work. This proof is difficult to generate because it can only be generated by trying billions of calculations per second. Miners need to run these calculations before their block is accepted and they are rewarded. As more people begin mining, the difficulty of finding a valid block is automatically increased by the network to ensure that the average time to find a block remains at 10 minutes. As a result, mining is highly competitive and no individual miner can control what is included in the blockchain.
The proof of work is also designed to rely on previous blocks, which enforces the chronological order of the blockchain. This design makes it extremely difficult to undo previous transactions because the proof of work of all subsequent blocks must be recalculated. When two blocks are found at the same time, miners process the first block they receive and once they find the next block, they forward it to the longest blockchain. This ensures that the mining process maintains a global consistency based on processing power.
Bitcoin miners can neither cheat to increase their rewards nor process fraudulent transactions that damage the Bitcoin network, because all Bitcoin nodes will reject blocks containing invalid data that violates the rules of the Bitcoin protocol. Therefore, even if not all Bitcoin miners can be trusted, the Bitcoin network is still secure.
