Affiliate marketing is when you promote (advertise) your partners product and send traffic to his site.
In most cases you earn commission when visitors buy that product but there are other programs too over which you make a commission.
When you simply drive traffic to your partners page (Pay Per Click) or to a squeeze page and visitors opt-in for his newsletter (Pay Per Lead) you will be paid after the number of visitors who signed up.
The advantage of affiliate marketing is that it's practically easy to register to an affiliate program and right after that you can put instantly your partners ad on your page.
After registration you'll receive a link (hoplink), different sized banners, email swipes, sales page content, and sometimes product descriptions too to put on your page.
If your visitor clicks on one of the ads and buys the product on the other side then you'll receive a commission which can be between 10% to a whopping 75% or maybe more. About the commissions you can read on your partners affiliate page before you even sign up.
From most websites and partners you'll receive the payment when you reached a certain amount and that's about $50 or $100 program and partner dependent.
A few big Affiliate Marketing Programs are: ClickBank.com, CJ.com, JVzoo.com. Here you'll find a bunch of products that you could promote. Just choose a category and read carefully the product reviews.
But before you advertise a product I recommend to research about it a little on forums, blogs, etc. See what people who tried that product are saying to find out if it's really a quality product or not. This will reflect back on your page.
You don't want to promote a bad product because you'll lose clients. I know, you'll say that it's the sellers problem if a product is bad and the customer is not satisfied, but how did that visitor end up at the sales page? From your ad, from your website! That is why it is important to know what are you selling. Yes, everybody wants to make sales, earn big commissions but for the long-term it's more important to have satisfied customers who will probably be back for more.
One other thing that you might consider is not to promote products from too many programs on your site. It would be best to choose a program that has a bigger variety of products because you can reach faster the $50 or $100 limit after which you'll be paid. If you choose one product from each program you'll reach that limit maybe much later and won't enjoy the fruits of your work that soon.