A miner is a person that does mining work. In Bitcoin's early days, it was possible to mine for Bitcoins on a personal computer. These days, however, it is not possible without operating dedicated clusters of compute farms, with the number of nodes often in the thousands.
About ten companies known as "mining pools" account for over 90% of mining, most of which are companies in China where electricity and startup investment are cheap. Their total investment is said to exceed several hundred billion yen.
Miners are said to hold generation rights to their share of the block.
This also counts as voting rights on changes to the Bitcoin protocol, which means that arbitrary changes that miners do not approve of are ignored and gives the greatest influence over Bitcoin protocol changes to those investors who invested the most.