Search Engine Marketing (SEM) and Search Engine Optimization (SEO) are two sides of the same coin. Where SEO is often much more of a marathon and can take some time to start reaping rewards, SEM can be and immediate gratification for your marketing efforts and dollars.
With SEM you pay-per-click to your ads. This pay-per-click method is often referred to as PPC. PPC ads can be placed on several search engines (like Google and Bing) and on their partners sites. This gives you an opportunity to get in front of potential customers in a variety of places and formats.