The way of making money online by marketing a company's or single person's business/service/product/website..
in another way, you will be rewarded by that particular company or person to market their business/service/product/website..
Example..
you can see in some websites they are marketing amazon products. if the website owner (webmaster) sell any items from his/her website then he/she will get commission from amazon.
sample website -
http://mobilespecs.info/ (you can see amazon products in this website)
another example is
Marketing file uploading websites (fileserve, filesonic, oron, etc). if you make any sales from your website or giving your affiliate link directly to people, then you will get commission.
and there are more (google for more ways of affiliate marketing)
It is a big area and if you do affiliate marketing in a good way. your bank account will be increasing while you sleeping as well..



but to do affliate marketing, you have to work hard through online and you have to know online marketing strategies (email marketing, SEO marketing,make campaigns, etc) and if you have a website with good traffic (atleast 500 visitors per day, you can do it easily by publishing affiliate links and banners on your website..
hope u will get an idea from this. for more search google..