Network marketing is a business scheme whereby you "recruit" users or sellers of the product and subsequently take a % of profit from the product's sales. Similarly, the person you recruit also can recruit more person under him or her and gain the same benefit. Commission schemes of different MLM (multi level marketing) company differ but their general principal are the same

Normally these business work by way of mouth rather than advertising, which is why you have no idea they exist. Amway by far is the largest and the most established network followed by many other. Some people do consider these business as pyramid scheme but in essence these company "convert" advertising dollar to revenue for consumers who participate in their scheme