A Pyramid scheme is when early adopters are paid by later adopters. New adopters are placed "under" the older adopters and form a pyramid because it takes more newer adopters to pay everyone above them.
If I go to you and say "I'll give you $20 next week if you give me $10 now", and then go to another person and do the same.. and in a week I have a bunch of money and I start paying the people who first paid me with that money, that's a pyramid scheme.
Eventually, you run out of people who are either unwilling or unable to keep putting money into it, and at that point you are unable to pay the later adopters anymore. So, you take what's left and head to a country without extradition.
Bitcoin is NOT that. Nobody is paying other people with the investments of the people that came before it.
Bitcoin is no different from any other currency exchange, and it's not that different from the stock market, with the exception that stocks are typically issued against assets, so if the stock tanks, then assets can be sold to pay back investors. That doesn't really exist in currency exchange (virtual or otherwise).
You can buy Euros with dollars, and wait for the value of the Euro to appreciate and sell them back. That's called ForEx (or Foreign Exchange).
