There are two main approaches to making money on your website.
1. Directly offer a product/service for sale at your website. You can offer a product or service on your webpage or you can offer free information to lure people to another where you then have a product or service available. This is the most popular way of making money on the net because it's easy to set up and you can get the initial profits rolling almost overnight.
Or,
2. Get as many visitors to your site as possible by enticing them to come with something special you offer (Such as valuable information, help, files, free stuff, etc.). Then, once the visitor counts are high enough, you can sell advertising space on your site or sponsorships or banner advertising.

The above aspect of online marketing, with the intent to sell advertising space, is not talked about as much because it doesn't generate instant cash and requires an investment of time and money of at least six months. Once these sites are successful however, they pay off big and the time commitment and maintenance from that point on is minimal. A good example can be found at http://www.theautochannel.com.
The autochannel site has been a success because it offers daily news on the automotive industry and free information on all things automotive (including new and used car prices and dealer invoice costs). To the best of my knowledge, the AutoChannel generates over three hundred thousand visitors a week (that's millions of hits a month) and can sell premium advertising space. They generate over $100,000 each month in advertising fees from corporations like Pennzoil and Allstate, regardless of what else they are able to sell.
Another extraordinary example of this is the Netscape site which gets over five million visitors a week and is now selling banner links for a whopping $35,000 per month. Just think; if you have just 30 pages, each displaying a banner that companies are paying you $35,000 per link per page ... that's over a million dollars each month in advertising revenue alone! Of course, the chances of you or me getting as big as Netscape are slim to none... but this example does show you the potential I am talking about.