An advertisement that is displayed by rank of popularity depending upon the result of the search being madefor the keyword that is linked to the listing through seacrh engines like Yahoo, Google,Excite, etc.
In this way, you can drive traffic to your promotion for having a high ranking in search results.
Many people acquire the information through the serach engines so this method of advertising is cost-effective.
Yahoo, J-listing and Look smart are the leading search engines that are providing SEM services.
Many people are searching for information by category so it won't necessarily mean that you'll achieve high rankings in search engines , listing by category should also be consider.