What is a miner?

Mining
Miners assemble the bitcoin blocks. First, they group the transactions that are being propagated in the network, but that have not yet been included in a block already connected with the current. Once the block is mounted, the miner calculates the hash for the block. But bitcoin does not accept any hash.
Calculating the hash of the same data will always have the same result. There is, therefore, a data within the block that the miner can manipulate. This data is just a number, called a "nonce". The miner then starts calculating repeated block hashes, changing only the nonce number at each attempt.
Since the hash is just a number, bitcoin establishes a maximum value for the hash. Bitcoin works with extremely large numbers, but for example, let's assume that the hash can not be greater than 8. When the miner finds a nonce that causes the block to have a hash of 6, for example, it propagates This to the network. The block is solved and can integrate the chain of blocks.
Bitcock blocks should take, on average, 10 minutes to have their "nonce" found. If the miners are finding the "nonce" very quickly, the maximum value (8, in the example) is decreased, to make it difficult to work. If the miners are very slow, the maximum value is increased, making the work easier, as the number of valid hashes increases. This adjustment is proportional and is calculated by the network periodically, based on the average generation speed of each block.
For their work, the miners earn a quantity of bitcoins. Currently, this value is 25, but it will be halved when the network reaches a certain number of blocks, until currencies are no longer generated. The miner also has the "change" of all transactions of the block, if that change has not been "returned" to the payer in the transaction