Bitcoin is a digital currency, first introduced on January 2009, although some people don't consider it as a currency. It's not issued by any central banks, new bitcoins are created by network, the process is called "mining". The mining nodes are computers on the network, they're awarded bitcoins each time when they solve a certain mathematical problem. The nodes are powerful computers, usually boundled together to meet the challenge in bitcoin mining. But difficulty to solve the problem is adjusted automatically with the overall stength of the network, more powerful, more difficult.
For the first four year (2009-2012), on average blocks are mined ervery 10 minutes, each block has 50 new bitcoins, so total 10,500,000 new bitcoins were created. The amount halves ervery four years thereafter, so from 2013 to 2016, new bitcoins created will be around 5 millions. From 2017 to 2020, it is around 2.5 millions. The total number of bitcoins will not exceed 21 millions.
Bitcoin is usually denoted as BTC, which is not an official ISO 4217 code yet. The smallest amount of a bitcoin is 0.00000001 BTC.
People can get bitcoins by accepting bitcoins as payment, purchasing bitcoin in a bitcoin exchange, or joining a mining pool. Bitcoin is so young, it might be very risky to invest on it today.