Cryptocurrencies are digital currencies with no central account and no legal backing. They have no paper representation and are independent of a central economy. These currencies are completely virtual. The bitcoin network by Satoshi Nakamoto came into existence in 2009. Since then other forks of bitcoin and cryptocurrencies have evolved.
To know whether bitcoin is good or bad, you need to know what it actually does. There are mixed reactions about this. The value of these cryptocurrencies aren't steady. They could rise fall drastically against the dollar. With Governments all over the world skeptical about bitcoin and other cryptocurrency, they are still being integrated. Transactions both online and offline all over the world are done with cryptocurrencies. I like to think of them as "unofficial currencies."
There are different methods of acquiring bitcoins and cryptocurrencies. You can buy them with the current price against dollars, claim from faucets or mine them using mobile and computer. They have wallets online where you you can store them all unique to individual currencies.