Definition: Search Marketing or Search Engine Marketing (SEM) is an activity one does using "Organic" (non-paid) and "PPC" (paid per click) strategies to drive a defined set of users to a defined site page for a defined reason. SEM includes both search engine optimization (SEO) and search advertising, or paid search.
What is SEM? SEM is the planning and effort to make your site more "findable" to specific keywords using SEO and Paid services.