Mining is the process by which Bitcoins are put into circulation. Miners perform mathematical calculations for the Bitcoin network with their "ASIC" computer calculators to confirm transactions and ensure their integrity. As a reward for their services, they collect the newly created Bitcoins as well as the fees for the transactions they confirm.
The "pools" or "miners' cooperatives" are in competition with each other and their revenues are proportional to the number of calculations performed. Moreover, the network is growing steadily, as is the difficulty in validating transactions so that the number of bitcoins generated is always the same, regardless of the overall power of the network.