New bitcoins are generated by the network through the process of “mining”. In a process that is similar to a continuous raffle draw, mining nodes on the network are awarded bitcoins each time they find the solution to a certain mathematical problem (and thereby create a new block). Creating a block is a proof of work with a difficulty that varies with the overall strength of the network. The reward for solving a block is automatically adjusted so that, ideally, every four years of operation of the Bitcoin network, half the amount of bitcoins created in the prior 4 years are created. A maximum of 10,499,889.80231183 bitcoins were created in the first 4 (approx.) years from January 2009 to November 2012. Every four years thereafter this amount halves, so it should be 5,250,000 over years 4-8, 2,625,000 over years 8-12, and so on. Thus the total number of bitcoins in existence can never exceed 20,999,839.77085749 and counting.
Blocks are mined every 10 minutes, on average and for the first four years (210,000 blocks) each block included 50 new bitcoins. The current reward for new blocks is 25 new bitcoins.