Bitcoin is the equivalent to a bag of cash, the value varies by day to day so you can't always tell what you can get. It's like a virtual stock market. The system is also P2P meaning it's non-centralized and very hard to track down it's users. That's why 'questionable' sites use it.
Edit: The reason this thing works is because it works the same way USD or any other currency works, it prints money and people get it. And inflammation will probably be unbearable in a couple years so enjoy it while you can.
