Multi-Level Marketing (MLM) is a form of direct marketing. The multi-level refers to the fact that (mostly) private individuals are recruited as sellers who receive commissions when they recruit others. The sales force structure looks like a pyramid. MLM is legal as long as they can credibly prove that their purpose is not simply to recruit salespeople (who are also customers).
The operators of MLMs like to say that it is the fastest marketing method for growth. That may be true, but only for the pockets of the operators themselves. The lower a salesperson is in the pyramid hierarchy, the lower the earnings. Not a single MLM company in the world has ever made it into the rankings of the largest companies. But there is always the danger that an MLM company is a Ponzi system.