Paid search advertising is a tool to get your web site to come up in top placements in the search engine results, appearing before what are called the organic placements. When a search term is entered into a search engine like Google, the search engine displays a list of web pages relevant to the search term. The list contains paid results, PPC ads, and non-paid results, the organic listings.
PPC ads are the listings that appear at the top, above the organic results. The PPC ads are labeled to distinguish them as an ad. In order to have your site appear in these top results, you engage in paid search campaigns through the search engines, for example Google Adwords in the Google search engine.