Affiliates are very closely related to advertisements. As opposed to pay-per-click and flat rates for ads, affiliates only give you money when someone becomes a member of their site directly from your site. You get a larger amount of money from affiliates than ads, but it takes a lot more follow-through from the user.
The bad aspect of an affiliation is that the user must register with the site for you to make any money. A lot of times this requires putting in information like the user's name or email address, and people can be very reluctant about giving information about themselves.
Affiliates provide you with pre-designed banners or text links to drive traffic and potential members to their site. I've got several affiliates on my site, but they're all from free paid survey sites on my survey page.
People have made money from affiliations, but for a beginning user it's tough to do so. Most affiliations are free though, so sometimes it may be worth a shot.
Warning: If you make a site that has ONLY affiliations, not only will you make no money, but you will be penalized by search engines. You must have CONTENT along with those affiliations or your site is useless, not original and just another of the trillion sites on the internet.
Some sites that offer affiliations for multiple sites (among other things) are:
Here are some examples of my affiliates:



Share on Facebook