Pay per Click Advertising

Most search engines have two sets of results on every webpage they show to users. The top results are normally paid (sponsor) results and the other results are called organic results.
 
Sponsor results are websites that have paid to be
displayed on the results page of the particular
keyword or keywords that are being searched.
Organic results are websites that have been optimized (or just got there by random luck) so that
they would be placed on the results page of a
particular term or phrase being searched by a user.
 
With PPC, you purchase the right to have your
site’s name and brief description appear as a link
alongside, above, or on the same page as regular
search returns. This strategy typically yields the
most consistent results and, when organized
properly, will help deliver more business from your website.
 
Market Hardware's Compass blog post