Multi-level Marketing is a perfectly legal business model. Pyramid schemes are illegal. The FTC and similar bodies around the work draw a clear distinction between the two. Some marketers use very inventive strategies to blur that line.
Both types of marketing can chug along making many people lots of money for an indeterminate period of time. They can both potentially fall apart when the limits of a finite market are reached or when people have their confidence shaken, leaving the vast majority of people in a losing situation. The illegal ones can come to a VERY abrupt end leaving the vast majority of people in a losing situation.
The FTC has a fact sheet advising people on how to identify and avoid Pyramid Schemes.