Search Engine Marketing
Search Engine Marketing (SEM) is also known as search engine advertising.
On sites like Google AdWords and Microsoft AdCenter, you bid for your preferred keyward against other advertisers. When these words appear in users search quearies, your ad appears alongside the search result – usually labelled as sponsored links or sponsored results.
Most SEM is on a pay per click (PPC) basis, under which you only pay when a user clicks through from an ad to your website. Thus SEM is very cost-effective, sine the ad is only triggered by relevent search terms and you only pay for actual traffic delivered to your site.
Google AdWords and Microsoft AdCenter both provide some online advice in choosing keywords and managing SEM campaigns.