Ponzi schemes

[Affiliatestech]

Can someone tell me what a ponzi scheme really means? There are lot of people on facebook asking me to join them in their business. When I see the reviews for those sites, they say the site really pays but its a scam because they uses ponzi scheme. I've been trying to understand the meaning of ponzi but I couldn't understand. So please tell me what ponzi really means and suggest me why not to invest on those schemes.