With MLM it is an actual legal business system that provides customers with an actual product or services. So, if you become involved in mlm, then you will be selling a product or service to someone in return for their money and this is what all legal businesses, online and offline do also.
Now that you understand the differences, you can see that mlm is definitely not a pyramid scheme. You just have to be sure that you take time to choose the right opportunity that is legal and not set up like a pyramid.