Most people know that about Google Adwords - they are the adverts that often appear on the right-hand side of Google search results. Companies usually create their own Google Adwords, unless they have big enough budgets and Google will help them create this, and they only pay when an advert is clicked on. In most cases the more you pay the higher you get, though companies who have written good adverts that are clicked on are rewarded by Google and can appear above those who have paid more.
It is worth considering what Jakob Nielsen (www.useit.com), the web usability expert, has to say on Google Adwords in an article he wrote way back in September 2001: ".. I have always maintained that search engines are the exception to the Web advertising rule. There are two reasons for this:
"Search engines are the only type of site that users visit with the explicit intention of finding somewhere else to go as quickly as possible.
? Because they know what users are looking for, search engines can target ads to a user's current navigation goals.
? Displaying an ad for something that the user immediately wants is much more powerful than targeting ads based on general user profiling and demographics. "
Most of the observations from the article - Designing Web Ads Using Click-Through Data - are still relevant today.