PPC, which stands for pay-per-click, is an online advertising model where advertisers run ads on a platform similar to Google Advertisements and pay a figure every time someone clicks on it. Run nearly any search on Google (or Bing), and you'll see advertisements displayed at the top of the results page.
PPC as a marketing channel covers a number of different advertisement platforms, with the most common of these being Google Ad Etc. Search Ads, Shopping Ads, Display Ads, Video Ads, Gmail Ads etc.