MLM == Multi-Level Marketing
Basically, pyramiding. I consider them to be something smelling like scams, but not quite there. Prices tend to be outrageous in some cases and the way to make money is to get people under you so you can get residuals from their work. As I wrote, not quite a bald-faced scam, but smelling a bit like it. OTOH, if you get in early you can make huge money. It's the people who come to the game late that take it in the neck usually, but there can be exceptions there as well. Most of the MLM ventures tend to go nowhere in the long run, but some fly - like USANA and Amway. IMO the value is dependent on one's personality - what you like, how you work, etc. It is not for everyone.