Miners are responsible for validating transactions, creating blocks and adding them to the blockchain. They do this by aggregating new batches of transactions (blocks) and proposing them to their full node peers within the network, adding them to Bitcoin’s transaction history (the blockchain). To propose valid blocks, miners must undergo a stringent process that expends energy and time and results in the generation of a Proof-of-Work, which is attached as part of each block and can be easily verified by full nodes running the Bitcoin software. Adding a valid block to the blockchain allows a miner to reward themselves with freshly minted bitcoin and claim all transaction fees offered by senders. This reward is given as compensation for the work incurred in finding a valid Proof-of-Work.
Becoming an effective miner requires advanced hardware and software well above the level demanded of other network participants (full nodes). Mining on the Bitcoin network has evolved from a geeky hobby into a fiercely competitive global industry, as miners compete to generate valid Proofs-of-Work and most efficiently claim the bitcoin rewards.