Affiliate marketing has been quite a popular subject on the Internet for a number of years because of its potential to generate big revenue for anyone who masters it.

But few people actually master it. Part of the challenge lies in the fact that millions are trying it.

For newcomers, an affiliate marketing strategy is a way to generate revenue for a Web site publisher by marketing the products and services of other companies. If a site visitor clicks on a text link or banner add, goes to that company Web site and makes a purchase, the originating Web site gets a commission on the sale.

A Web site publisher signs up for an affiliate marketing program with any number of companies that specialize in it. The affiliate marketing companies represent the advertisers, who provided the links and banners. When the publisher put those ads on his or her site, they include a tracking code that allows the publisher, the affiliate marketing company and the advertiser to track the number of visits and purchases and give proper credit to the publisher.

Success is measured with three numbers in particular. First is the ad impression, second is the number of clicks on the impressions and third is the purchase.

Two rates are derived from these metrics -- the click-through rate and the conversion rate (also known as the conversion ratio). The click-through rate is the number of clicks divided by the number of impressions. The conversion rate is the number of purchases divided by the number of clicks. There are no known published averages for either rate.

One major affiliate marketing company quoted an average click rate of 1.9 percent and an average conversion rate of 3.2 percent. That means out of every 1,000 impressions of a link or banner, 19 people would click on it and 0.6 would make a purchase. The average purchase was about $47.50. Commissions vary from one product or service to another, but a 10 percent commission on that average would generate $4.75 for the publisher.

It is worth mentioning that click and conversion rates vary widely from one site, product or service to another.

Affiliate marketing has its place on many Web sites, but it is best used as a secondary income stream to complement banner ads and other revenue sources.