Most search engines utilize PPC (pay per click) or PPM (pay per impression) systems to generate an income and allow a site to gain a lot of exposure.

Pay Per Click is an auction based system that allows you to bid for placement on a search results page.  A user of a search engine will type in a few keywords to find what they need.  As an advertiser, choosing keywords that will trigger an ad is the first step to getting an ad seen.  The goal of a PPC for a search engine is to show the most relevant ad based on the search just like in the free search results.   When the user clicks on the ad and visits the site being advertised, the advertiser pays an amount for that click.   Depending on competition, region, ad quality and other factors will determine how much that click will cost the advertiser. 

Pay Per Impression (PPM) is roughly the same except that a user doesn't have to click on the ad.  The ad just has to be displayed on the page so that the user will see it.   These systems are normally used on content based websites and only used on search engines if the click rate on the ad is zero.  They are normally set in blocks of 1000.

Hokorawa specializes in this type of advertising being a Qualified Google Adwords, Yahoo and MSN specialists.   These three search engines are the primary systems we utilize because of the ability to target the ads not just to keywords but to smaller demographics like, zip codes, gender, age and interests.

There are some community websites that have huge amounts of traffic that have their own ad systems.  These include MySpace and Facebook which allow you to target your ads down to the interests of the users and what the user does on the site.