SEM stands for Search Engine Marketing.
When someone is looking for the type of product or service you provide, they will use their favorite search engine (Google, Bing, Yahoo or one of the countless others available today). They will type in one to several words that they think best describe what you do or offer, and will then hit Enter.
SEM in part means marketing your web pages trough a paid CPC (Cost-Per-Click) or PPC (Pay-Per-Click) marketing plan. If you are running an SEM campaign and are paying for PPC, the relevance of your online advertisements to the search query a user performed will determine where you show up on the sponsor links section of the search page, how much you are paying per click, and what you are spending each month for that campaign. This is referred to as a paid listing. When you use Google, Bing, Yahoo or other search engines to perform a search, you will usually see the organic listings in the left column, and the paid listings on the right column (or sometimes in the top several rows on the left above the organic listings.