The idea of associate or affiliate marketing programs was first made popular as an internet marketing strategy in 1996 by Jeff Bezos, the chief executive of Amazon. In this type of marketing, an online business pays people with other websites to send them traffic. Simply put, a link to the business is put on the affiliate's site. If that link takes traffic and or business to the parent site, the affiliate is paid a pre-agreed commission. It is, therefore, in the affiliate's interests to do whatever they can to promote their own site and the links in it. This is easier if the website is already in the same field as the parent company.
If, for example, you have a website and all of your content is concerned with cars and their maintenance, don't sign up as an affiliate with a site that has absolutely nothing to do with motor vehicles. The whole point of associate or affiliate marketing is to attract traffic to your own site because the person searching is interested in your content. If the links to the site that you are affiliated with are on that same subject, the visitor is more likely to click on them. This is especially the case if your content relates to the products on offer at the affiliate site in an indirect way.
Good associate/affiliate programs are out there. There are some unscrupulous people who either don't have a product or don't pay the commission for one reason or another. Plenty of them, though, are honest. The best ones will give you the information that you need to be able to start making money. To begin with, stick with one affiliate. Some people get so taken with the idea that they sign up with half a dozen. Unless you have half a dozen websites, this is counter productive because they won't all match with your content.
Some sites will actually advise you not to sign up with other sites until you have made the first one successful. Arguably, these are the best sites to become associated or affiliated with because the advice that they are giving you is good advice. Another thing that a quality program will do is to advise you to use pay per click. Pay per click relates to the keywords you choose to use to promote your site - generally based on what search terms you think people will use to find your content or the product you are associated with. You then tell the pay per click vendor what you are prepared to pay when someone clicks on a particular word so you might offer 0.8 cents for legal services if the law was your interest. You can generate income from affiliate marketing if you put in some work, get involved in promotion and tailor content and product.