The three primary pay per click providers are Google Adwords, Yahoo! Search Marketing (Overture) and Microsoft AdCenter. Each is similar in the sense that you pay a fee to be listed in advertising sections of search results on varying search engines. Where they display results and the process of getting listed differs in each. Yahoo Search Marketing is much more direct. You simply bid on keywords or phrases and make a bid and your site will be displayed in an order based on on the highest bid for each particular keyword/phrase. Google Adwords is somewhat more complex and is based on a combination of factors including maximum bid, daily budget, click ratio, and more.

See below for information on each:

Google Adwords

Strengths
Google AdWords has by far the widest coverage of any of the providers, allowing advertisers to reach a wide market using just one system. This coverage also makes it the most suitable for specialised businesses, who would not receive much search traffic from the smaller PPC providers. The minimum cost per click for a keyword can be a little as 1p, which can make AdWords suitable for editorial or low order value websites, where the 10p minimum cost per click required by the other providers would not be so cost-effective.

Weaknesses
Google AdWords has now become so popular in some market sectors so that bidding is very competitive, meaning in some cases better value can be found elsewhere. The system provided by Google is very sophisticated - for example, ranking position can be determined by the bid price and the clickthrough rate of the advert - so that the service can be difficult for beginners to understand initially.

Yahoo! Search Marketing

Strengths
Yahoo! Search Marketing provides coverage within the search results of Yahoo! and MSN, which each hold around a 15% share of the UK search market. Larger text adverts are allowed than on Google AdWords, enabling a user to consider your offering in more depth before clicking on your advert, which we have found can result in a higher conversion rate.

Weaknesses
Yahoo! Search Marketing reviews many adverts manually before advertisers can use them, which, combined with an aging management system, means that it can be cumbersome to make small changes and test different versions of an advert on an account. For smaller advertisers the £20 per month minimum spend requirement can mean this channel is not cost-effective compared to Google AdWords. MSN AdCenter

Strengths
The format of Microsoft's PPC service is similar to Google AdWords, but it also provides an added level of campaign targeting and management. By using the data from MSN's numerous registered users who have signed up to Hotmail, Passport and Messenger services, Microsoft adCenter will use this data, when available, to provide demographic data to their PPC search market management and response data. Advertisers can place higher bids to boost their listings if searchers match their selected market profiles and although this data won't always be available, it could prove useful.

The other main advantage offered by the Microsoft adCenter is the ability for 'day parting', whereby you can stipulate at what time of day your adverts appear, or on which days of the week if these can affect the responsiveness of your campaign. This can potentially give the advertiser much more control over their spend and a greater ability to test the responsiveness of their advertising programme.

Weaknesses
Conpared to Google, Microsoft only has a small share of the search market, so activity here may may not be significant for niche advertisers. There are also a range of usabilty issues to be ironed out in the AdCenter interface.