When a new block is discovered, 25 BTC is awarded, and split up to everyone who has helped in the mining process. Because the difficulty of mining is so high right now, miners mine in a pool, so the rewards gets distributed accordingly to how much you contributed.
On top of that, miners get paid with our transaction fees, which will increase when difficulty increases, for helping us verify the block. That's how they earn bitcoin from mining. Of course, if you can verify a block all by yourself, you can get 25BTC.