SEMPO (Search Engine Marketing Professional Organization) define PPC advertising as
“a model of online advertising in which advertisers pay only for each click on their ads that directs searchers to a specified landing page on the advertiser’s web site.” (2011)
Currently the three main channels for PPC are Google, Yahoo & Bing, although this will change during 2011 in the UK & Europe when all Yahoo search results will be powered by Bing (the change has already taken place in the US). There are also a number of other media operators offering pay per click advertising models including Facebook & LinkedIn (DirectAds).
The pay per click model is based on a bid system where advertisers bid on keywords or search queries that are relevant to their website. When an advertisers keyword is triggered a relevant ad is then shown. The advertiser is then only charged when a searcher clicks through, landing on the site in question.
Pay per click advertising is sometimes known as sponsored listings or paid search and is one of a number of tools available to search engine marketers, along with search engine optimisation (SEO).
You will find the pay per click ads on Google across the top of the search results (SERPs) as well as down the right hand side of the natural listings.
For any commercial enquiries, please visit the Screaming Frog site where I can get back to you.