Affiliate marketing is the process of promoting another company's products or services for a commission.
This is one of the ways I've been making money on the Internet since 1998.
I love affiliate marketing because it's a very simple, transparent model for earning an income online. It's not about joining some pyramid scheme where you have to recruit people to sell random products (I really hate those programs.)
You create a useful, themed website that provides information, and then casually recommend products that are related to your topic.