Lately, all eyes have been on Herbalife, a $4 billion weight loss and nutritional supplement company that hedge fund manager Bill Ackman has attacked as an alleged "pyramid scheme."

Herbalife contests those charges, saying its multilevel marketing strategy is above board. But the headlines have brought this huge industry into the spotlight.

From lipsticks sold by iconic "Avon ladies" to the popular P90X workout system, multilevel marketing companies represent nearly $30 billion in annual U.S. sales and enlist tens of millions of Americans as independent salespeople. Here's a look at how the industry works.

What is multilevel marketing?

Multilevel marketing companies employ a network of independent salespeople who sell products directly to people in their community. These salespeople earn income based on their personal sales, as well as the sales of people they recruit to work for the company.

While critics accuse multilevel marketing companies of being pyramid schemes, the Federal Trade Commission says that a company only qualifies as a pyramid scheme if its salespeople are paid primarily on the basis of recruitment, as opposed to the sale of a retail product or service.

Related: Why Bill Ackman is targeting Herbalife

"While we certainly rely on recruitment to sell our products, the compensation is based not on the recruitment of other people but ultimately on the sales of the product to real people who are using the products," said Joseph Mariano, president of the Direct Selling Association, an industry trade organization that represents roughly 200 direct sales companies, including Herbalife. Nearly all direct sales companies rely on a multilevel model.

How does it work?

When you sign up to be a salesperson for a multilevel marketing company you likely need to purchase some of its products upfront so you'll have inventory to sell.

The company will offer products to you at a wholesale discount and then you sell those products to friends, family members and other customers at a higher retail price. If you convince a customer to become a salesperson, then you will earn income off their sales as well. Same goes for anyone they recruit. And so on. As you and your recruits sell more products, your earning potential grows.

Top sellers are often rewarded with additional incentives — from free company cars (think Mary Kay's pink Cadillacs) to all-expense paid corporate trips.

However, there are a lot of out-of-pocket expenses that can quickly add up. Besides purchasing the inventory, there are registration fees, welcome kits, training seminars, marketing materials and product parties.

In order to sell phone service for ACN, for example, sellers must pony up a $499 application fee. To become a Mary Kay beauty consultant, you have to buy a $100 starter kit.

And most companies have a laundry list of requirements, including sales quotas of hundreds or thousands of dollars a month, that must be met to stay an "active" seller and to qualify for coveted commission checks.

Who are the salespeople?

In 2011, roughly 15.6 million Americans worked as salespeople for multilevel marketing companies, according to the Direct Selling Association.

In the past, the typical direct seller was a middle-aged woman, and the DSA's data still backs that up. But things are starting to change.

While a vast majority of salespeople -- 78% -- are still women (many companies like Avon and Mary Kay target women almost exclusively), the percentage of men who are entering the business is growing. In 2011, about 22% of salespeople were men compared with 14% in 2008.

And the ages of sellers are starting to skew younger as well, according to the DSA.