What is Bitcoin?

Bitcoin is a type of cryptocurrency. It is the first decentralised digital currency.
Like real money (or fiat currencies), Bitcoin can be used to store value and to buy things electronically.
What makes Bitcoin different to fiat currencies like the Dollar or the Euro, is Bitcoin is not printed on paper and has no central banks that regulate it.
Bitcoin is decentralized. This means that the ledger / database that records who or which Bitcoin address owns how much Bitcoins, and the transactions - i.e. how much Bitcoins was transferred from which account to which account, are spread across the Bitcoin blockchain network.
Bitcoin can be transferred peer-to-peer without the need of a 3rd party trust (like banks or credit card networks). This is made possible with the distribution of ledger using blockchain, in which identical transaction records are kept by multiple machines (hundreds or even thousands) on the Bitcoin network.
The machines / ledgers that record Bitcoin transactions are called miners. These miners are being rewarded with a small number of Bitcoins for their work.

How Much Bitcoin Supply are There in Circulation?

At the time of writing (25-Mar-17), there are 16.2 million Bitcoins in circulation.
A number of Bitcoins are kept being issued regularly. But because the additional supplies (mining rewards) are always halved for the following blocks, the supply growth in total will only move closer to 21 million Bitcoins, without actually reaching and passing it.

If you are based in North America, most countries in Europe, Australia or Singapore, you can buy Bitcoin via Coinbase. Buy using this link to get US$10 bonus in Bitcoin, if you buy or sell US$100 of digital currency or more.
If you are based in Indonesia, you can buy Bitcoin via Bitcoin.co.id.

There are many ways to keep your Bitcoin. However, if you do not control your own private key, that means you do not truly control your Bitcoin.

Using hardware wallets like Ledger and Trezor, is arguable the safest way to keep your Bitcoin.

Ledger Nano S is one of the most popular hardware wallets in the market, and it is not that expensive.
Ledger Blue has a better user experience and a larger screen, if you have some extra money to spend for your cryptocurrency hardware wallet.
Trezor is one of the first pioneers for a safe Bitcoin hardware wallet.