While "make money online" is a niche. E-books is not. A niche is a group of people who have a problem and are willing to pay for a solution. For example, golf is a niche that's part of the sports industry. Weight loss is a niche that's part of the health and fitness industry. Dating and relationships is a niche that's part of the self-help industry. Automotive is an industry that's all about the vehicles people drive. However, people can drive cars, trucks, vans, jeeps, and so on, Each one of those vehicles is a niche. Do you see the difference between an industry and a niche now?
When you get into affiliate marketing, you ultimately need to have a blog on your own domain and an e-mail list, but before you do any of that, you need to figure out what niche you want to be in. It should be a niche you like and already know something about (will make it easier for you to write content and connect with your audience). Making money online also shouldn't be the first niche you go into. Yo should choose a niche outside of it. Why? You won't know what a legitimate make money online program is supposed to teach you until you've had success with it.
When you're thinking about niches, think about magazines. They are already broken down into niche markets. I'll give you one last example. When I first started making money online and had to think of a niche, the first niche I choose was the learn Spanish niche. While the foreign language-learning market isn't as big as others, I choose it because I was a Spanish major in college. I understood the people and the products in the niche because I was part of it. When it came time to write content, I was basically able to just type from my head.
Joey
