Bitcoin and its other cryptocurrency competitors are in a nutshell digital cash.
Bitcoin is not backed by a government, organization or company.
Their value is entirely determined by the market for said coin at any given time.
Bitcoins are generated when computers solve a complex math problem.
Every time a Bitcoin is generated the problem becomes harder to solve and thus requiring more computing power.
Bitcoins aren't physical coins and only exist on digital storage mediums like USB drives and hard drives.
There is a set limit on the total number of Bitcoins that can be generated at 21 million.
There are currently 16 million+ bitcoin in circulation.
The New York Times goes into some more detail about Bitcoin here:
The record of all Bitcoin transactions that these computers are constantly updating is known as the blockchain. Why do criminals like Bitcoin? Criminals have taken to Bitcoin because anyone can open a Bitcoin address and start sending and receiving Bitcoins without giving a name or identity.
With the popularity of Bitcoin, competitors have emerged in the world of cryptocurrency. You can learn about them here:
Bitcoin has not just been a trendsetter, ushering in a wave of cryptocurrencies built on decentralized peer-to-peer network, it's become the de facto standard for cryptocurrencies. The currencies inspired by Bitcoin are collectively called altcoins and have tried to present themselves as modified or improved versions of Bitcoin.