The concept of affiliate marketing takes only a minute to understand. You earn a commission when you promote products for someone else. It's that simple.
The part that's confusing and challenging is the marketing. Because there are several ways you can market online:
Article Marketing (Hubpages, Squidoo, Ezine Articles)
Social Media Marketing (Facebook and Twitter)
Any time you are writing content, you have to take into account search engine optimization: keyword research, backlinks, ranking, niche market, niche competition, etc. That's more labor-intensive.
Beginners can lose money and waste time because they are overwhelmed by all of these techniques and information overload when they go at it alone.
It's much better if you invest in a training program and learn the basics. They're not expensive. They've got good ones online right now. Video tutorials and all that.
To caveat all that, I will say this: If you specialize in a niche that doesn't involve biz opportunities or make money online programs, like if you promote shoes from Amazon or something like that, the chances of you making money are higher.
Some internet marketing coaches may specialize in one technique over an another, but that's fine. As long as you understand the concept and what they teach you, then you're ahead of the power curve.
I wrote a Squidoo Lens about one training course that, in my mind, has the best teacher hands down for teaching newbies about affiliate marketing: http://www.squidoo.com/Internet-Marketing-Training...
If all you can do with a computer is email and surf the net, this guy is the teacher for you. I highly recommend him.