Search Engine Marketing
SEM is the short for search engine marketing. It includes both - gaining visitors through search engine optimization and through paid search advertizing. The latter is also referred to as paid per click (PPC). SEM gives advertisement for highly targeted audience on search page result and the advertiser pays only when the ad is clicked, that is, the advertiser's page is actulally visited. So you may say that SEM is performance based.
If you look at Google search result page, top right and left will show you search results that are achieved through SEM - the paid search marketing. SEM is time efficient compared to SEO. It is uploaded in hours and gives result in days. If you think you will use internet for your marketing you must think of SEM. Both natural and paid listings are important for online marketing and if you are not considering SEM you are going to miss a massive opportunity. New websites get more that 80% visitors via search engines. The trend in search engine results, for example in Google, shows gradual increase in paid results and gradual decrease in organic results. So in an advertising campaign you must consider SEM as one of the top priorities.