Paid advertising on a search engine results page (SERP). This is also called paid search. Search engine marketing (SEM) ads are used to drive traffic to websites but can include other calls to action, such as making a phone call or visiting a local store. Keywords in a search query trigger SEM ads. They usually appear at the top of search results and sometimes to the side. Most SEM is pay-per-click (PPC), so you only get charged if someone clicks on the ad.