Search engine Marketing; allows a business to advertise on major search engines using paid advertisements. These ads appear at the top, right side or sometimes bottom of the search engine result pages (SERPs) and are clearly marked as sponsored ads. Apart from the search engines, these ads can also appear on networks of associated sites.
PPC ads are based on bidding system wherein advertisers bid to have their ads listed on search engines. The bidding is for the search terms which are relevant to your business. The bid amount is payable only when a user clicks on the ads, thus giving it the name pay per click. Generally the higher you bid on a term the higher your ads appear on the search engines but Google uses a formula to give some benefit to the quality and relevancy of ads thus basing your position on more factors than just bid amount.
Major pay per click search engine advertising networks are: Google Adwords, Yahoo Search Marketing and Microsoft adCenter.