A content-rich site is one that has lots of informative articles up at it, usually centered around a theme. Most sites can't quite pull off being Wikipedia, so they specialize. For instance, you could do a site for dog owners. Possible articles on that site would cover:

• How to figure to what kind of dog you want
• where to get a dog
• How to deal with a puppy 
• Life stages of a dog 
• House-training puppies
• Dog training 
• What to feed dogs
• Whether to get more than one dog
• How to socialize dogs with other dogs and with cats
• Exercise needs of dogs
• Training dogs to do tricks
• Treating fleas 
• Common dog ailments and when to go to the vet
• Dog nutrition 
• Taking your dog on a trip
• Getting a pet sitter or boarding your dog if you don't take him on a trip

The articles you'll want to have on your site should be short enough so that someone can read them in about 5 minutes. This means you want to stick to articles of 250 to 750 words, with 300 to 600 words optimal. To give you an idea, a single page in a published novel has about 300 words.
          Of course, your real purpose in putting up all these nice little short articles and changing them out frequently is not to just put information out into the word. It is to have a site that people will come back to so that they will see the Google Ad Sense ads, and click on them, and then you will get checks in the mail.
           That's a key point, so I'm going to repeat it:

           The purpose of having a content-rich website is to attract people to it, again and again, so that they will see the ads and click on them.

           You might think it's lots of cool graphics and colors that make a site attractive to a visitor, but it's really the content. In order to make money from Google Ad Sense, you need to grasp that concept. Believe me, you could run a wonderful advertising campaign and develop all sorts of viral marketing tools and attractive affiliate programs.
           But unless your website is content-rich, the traffic spikes that you get for your efforts will only be temporary. The very best way to attract and retain an online audience is to provide content that's useful, valuable, informative, educational or just downright funny as hell or entertaining in some way.
            What does a content-rich website look like? Here are a few examples.