When you search for a keyword on Google you are presented with a list of websites that Google believes are relevant to your search. Within these search results you are presented with both organic listings and listings that are paid for. As a general rule the top 2/3 listings that appear at the top of the page (usually with a faint yellow background) and those search results that are displayed down the right hand side of the page have all been generated by paid advertising.
These adverts are being paid for in a system that Google calls Pay Per Click. In brief, PPC is a quick advertising means whereby successful adverts that have a high quality score (ads that are relevant) are displayed on the first page of Google, resulting in an increase in traffic directed to your website.