The most widely used and emulated pay-per-click advertising platform is Google AdWords, a bid-based system where you bid against other advertisers for space. Ads show up in Google’s search engine results pages, network partners and optionally, other websites, soon after you post them. Visibility is basically based on keywords chosen, budgets you assign and the click through rate of your ads.
Setting up a PPC campaign is simple and can grow into a complex management challenge depending on the competitiveness of the keywords and key-phrases chosen. Ads appear rather quickly and can provide that much needed traffic while waiting for organic listings to gain visibility.
Each search engine has structured their program in different ways. Although there may still be some bid for position models where the highest bidder gets the top position, Google set the current standard by taking many factors into consideration and developing a quality score to determine ad placement. Click though rate, maximum bid and even landing page relevance are all taken into consideration.
It looks and sounds easy, however it does take due diligence to get it to really perform for you. Best thing is that Google provides excellent training and explanations.