Unlike normal (fiat) currencies, bitcoin doesn't have a central bank or a backing of a national government. Instead, it is regulated by complex algorithms with new coins being mined when computers solve complex equations. All this number crunching is tracked by the blockchain, a public ledger of all transactions which is distributed meaning there is no central point of the network that can be shut down. It's perhaps analogate to file sharing protocol Bittorrent.
You can obtain bitcoin by mining them yourself of buying them from an exchange but for that to happen you will first need a wallet. There are different ways to get a wallet, you can simply store yourself on your hard disk in wallet software running locally or you can use online service and have your coins stored in a cloud. Both have advantages and disadvantages. E.g. if your coins were stored locally, if something was to happen to your computer such as your hard disk getting destroyed you lose your cash. Conversely, if your coins are stored online in an exchange there are more likely to attract hackers.
Famously in 2015, Mt. Gox, which was one of the largest bitcoin exchanges was hacked resulting in the virtual highest of 450,000 bitcoins with wooden valued at about 450 million dollars. While some coins have since been recovered, a large number are still missing due to the way the bitcoin protocol works, it's simply wasn't possible to hit the undo button. So, choose your wallet wisely.
