A miner is a person or company that runs special hardware to add new blocks to the blockchain. In return they earn the block reward: newly created bitcoin plus transaction fees.
What a miner actually does
A miner collects transactions from the mempool, builds a candidate block, and then hashes its header over and over with SHA-256. Each attempt changes a small number called the nonce. The goal is to find a hash below the target set by the difficulty. It is like a lottery where every hash is a ticket. Whoever finds a winning ticket first broadcasts the block, and nodes check it.
From laptops to warehouses
In 2009, Satoshi and other early users mined on ordinary computers. Then came graphics cards, and later ASIC chips designed only for Bitcoin mining. Today most mining happens in large facilities near cheap power, such as hydro, wind, stranded natural gas or surplus grid energy.
Mining pools
A single small miner might wait years to find a block. Pools combine the hash power of many miners and share rewards based on contributed work, giving each member a steady income. Pools coordinate the work, but individual miners can switch pools at any time.
What miners can and cannot do
Miners choose which valid transactions go into their blocks, and in what order. They cannot create extra coins, spend other people's bitcoin, or change the rules. Blocks that break the rules are rejected by nodes. A miner with a majority of hash power could reorder recent blocks or censor transactions, but doing so would be extremely expensive and would undermine the value of the coins they are paid in.
Miners are competitive businesses. Their income depends on the bitcoin price, fees, the halving schedule and the cost of electricity.