"PPC" is short for "Pay-per-click advertising." PPC is the practice of pushing ads to web users through a variety of paid channels. Ads are shown to users based on their search criteria and the advertiser pays for the ad only when clicked.
Note: PPC advertising is different than paying for impressions on a cost per thousand (CPM) basis. Cost per impression (CPI) advertising, more commonly used for branding a message, is billed based on increments of 1,000 impressions (hence, cost per thousand - CPM) and captures maximum impressions from your target demographic.
The following three (3) screenshots show paid ad placements on the may three (3) search engines: Google, Bing & Yahoo.
Example Advertising Placements on Google.com
Example Advertising Placements on Bing.com
Example Advertising Placements on Yahoo.com