New bitcoins are created through arithmetic operations on software—they are developed, instead of printed by a central government. No single individual or agency has the power to make up bitcoins. “ Mining” is the only root resource for bitcoins.
In its essence, Bitcoin is an triple-entry bookkeeping system open to all, while mining is fighting for the right to record on the accounting book. Miners collect and validate the information of the ongoing transactions, which have not been confirmed by the network. These transactions will be packaged and classified into a block and be written into a general ledger called blockchain. The block will remain a completed trading record and never to be tempered with.
Every 10 minutes there will arise such a block in Bitcoin world. All the mining computers around the world are trying to pack the confirmed transactions data in the last 10 minutes into one block and write it into the blockchain. The quickest miners will be the winners.
The miners, aka the winners who have successfully recorded the block, can get i) a sum of fee paid by the parties concerned, plus ii) an extra reward by Bitcoin network. The extra rewards are the new bitcoins developed automatically by the Bitcoin system. This is also the way bitcoins are produced. Thanks to the similarity between this mechanism and mining in the real world, these program developers are called as miners.
According to Nakomoto’s initial design, such a block is written into the blockchain every a certain period.. At the very beginning, 50 bitcoins will be created as rewards for the miners each round. Every 4 years the number of the newly-yielded bitcoins will be halved. Thus in 2140 there will be altogether 21 millions bitcoins produced. Although there are only a fixed number of bitcoins, every bitcoin is divided into satoshi, or 100,000,000th of one Bitcoin.
You can read more facts about mining here.