To understand paid search advertising, you must first understand a search engine results page (SERP). When you type something into a search engine (like Google), the search engine responds with a list of web pages (their title, links, and description) that are relevant to your query. This is your SERP. The bulk of that list is comprised of organic results: web pages that the search engine algorithm determined were the best match for your search.
PPC ads are the results (web page names) that appear at the top and bottom of your SERP. While we often think of ads as visually distinct elements, SEM/PPC ads look similar to the organic listings between them, but are labeled in some way to distinguish them as an ad. Here’s an example of an Adwords ad, which is the paid search ad for Google.