Satoshi Nakamoto, the unknown inventor of Bitcoin, stipulated the maximum number of Bitcoins ever in existence to be 21 million. The Bitcoins are added to the money supply through Mining. The first Miner who confirms a Block (of transactions) before anyone else gets rewarded. The rewards started at 50 Bitcoins per block in 2009 and is halved every 4 years. ~16 M Bitcoins are already mined and the remaining will be mined by year 2040. However more than 90% (19 Million) of the total supply will be mined within the next 5 years as shown below (the numbers are approximate).
In addition to new bitcoin award, Miners also receive transaction fees from all the transactions included in the block. After all the coins are mined, the Miners will still be motivated to continue mining to receive the transaction fees.
Good thoughts