PPC stands for Pay-Per-Click - a popular advertising technique on the Internet. Found on websites, advertising networks, and especially on search engines, PPC advertising involves sponsored links that are typically in the form of text ads. These are usually placed close to search results, where an advertiser pays a particular amount to visitors who click on these links or banners and land on the advertiser's web page.
In essence, PPC advertising is all about bidding for the top or leading position on search engine results and listings. Advertisers do this by buying or bidding on keyword phrases that are relevant to their products or services - the higher the bid, the higher the spot on the search results, the more the people will find the ad (and click on it) to go to their websites (this is why some people call it "keyword auctioning"). Advertisers would then pay the bidding price every time a visitor clicks through the website.