Google's advertising platform was launched in 2000 and is the biggest and most successful PPC system in terms of market and ROI. With Google adWords, their contextual advertising means you only pay for clicks and/or impressions. It offers a variety of features, including placement targeting, geo-targeting, multiple ad formats, and more. You can measure the effectiveness of your campaigns through tools such as conversion tracking and Google Analytics. Every aspect of your ad campaign is controlled by you and is fully customisable.
Yahoo! Search Marketing
Yahoo! took over GoTo.com's pay-for-placement search service in 2003 and renamed it Yahoo! Search Marketing. Over 2.3 billion searches occur on Yahoo! each month, a massive market to advertise to. Yahoo! provides many of the same features as Google AdWords, such as geo-targeting, ad testing, campaign budgeting, and campaign scheduling.
Microsoft AdCenter
The youngest of the three, Microsoft search advertising was launched in May 2006 and has a potential audience of 99 million search users. It aims to makes your search advertising campaign easy to manage, with simple-to-use in-depth campaign analysis; intelligent targeting based on location, gender, or day (although there is an additional cost for these specifics); and access to up-to-the-minute research.