Bitcoin, Litecoin, Ethereum, and other cryptocurrencies don't just fall out of the sky. Like any other form of money, it takes work to produce them. And that work comes in the form of mining.
But let's take a step back. Satoshi Nakamoto, the founder of Bitcoin, ensured that there would ever only be 21 million Bitcoins in existence. He (or they) reached that figure by calculating that people would discover, or "mine," a certain number of blocks of transactions each day.
Every four years, the number of Bitcoins released in relation to the previous cycle gets reduced by 50%, along with the reward to miners for discovering new blocks. At the moment, that reward is 12.5 Bitcoins. Therefore, the total number of Bitcoins in circulation will approach 21 million but never actually reach that figure. This means Bitcoin will never experience inflation. The downside here is that a hack or cyberattack could be a disaster because it could erase Bitcoin wallets with little hope of getting the value back.
As for mining Bitcoins, the process requires electrical energy. Miners solve complex mathematical problems, and the reward is more Bitcoins generated and awarded to them. Miners also verify transactions and prevent fraud, so more miners equals faster, more reliable, and more secure transactions.
Thanks to Satoshi Nakamoto's designs, Bitcoin mining becomes more difficult as more miners join the fray. In 2009, a miner could mine 200 Bitcoin in a matter of days. In 2014, it would take approximately 98 years to mine just one, according to 99Bitcoins.
Super powerful computers called Application Specific Integrated Circuit, or ASIC, were developed specifically to mine Bitcoins. But because so many miners have joined in the last few years, it remains difficult to mine loads. The solution is mining pools, groups of miners who band together and are paid relative to their share of the work.