Definition of Directs Sales from http://www.internetmarketingdefinitions.com/DirectSales
Direct Selling is the sale of a consumer product or service, person-to-person, away from a fixed retail location. These products and services are marketed to customers by independent salespeople. Depending on the company, the salespeople may be called distributors, representatives, consultants or various other titles. Products are sold primarily through in-home product demonstrations, parties and one-on-one selling.
Source: Pyramid Scheme From Wikipedia, the free encyclopedia
A pyramid scheme is a non-sustainable business model that involves promising participants payment, services or ideals, primarily for enrolling other people into the scheme or training them to take part, rather than supplying any real investment or sale of products or services to the public.
As you can tell just from those definitions, direct sales is about selling a product, so whenever money is exchanged, the person giving the money receives something in return. In a pyramid scheme, typically money is exchanged and there is not much product involved. You also only tend to make money when people sign up instead of when you make a sale.