SEM, search engine marketing, is not to be confused with scientific SEM, A scanning electron microscope. Ironically both terms refer to completely different processes although arguably both have their own unique levels of complexity. SEM is primarily used by an online business to continuously improve and maintain their website ranking as high as possible on the search engine results pages (listing of web pages that is displayed by a search engine in response to a keyword search) so that they can sustain a high level of inbound web traffic, and ultimately sales.

The terms SEM and SEO are frequently used interchangeably in internet marketing jargon and very often are assumed to b e the same thing. Whilst both are employed to ultimately improve a website's ranking in search engines, the differences are subtle and tend to get blurred very easily.

Fundamentally the main difference is that SEO tends to refer to a variety of techniques that are free (organic) and involve time and effort application, for example article development incorporating relevant keywords, establishing back links with other websites, forums and blogs, social media exposure in particular Facebook and Twitter, and blogging from one's own website on a regular basis.

SEM on the other hand is often described in one of two manners, either as a holistic umbrella term that incorporates SEO as well as paid advertising and all other search-engine techniques that drive increased traffic to your website, or it is portrayed as being separate and distinct from SEO, in that it refers to the exclusive commercial approach of internet marketing that involves paying for online advertising in one form or another, generally through using appropriate Ad Words applying pay-per-click (PPC) advertising, cost-per-thousand (CPM) advertising, or normal contextual site-specific advertising using text, banner or visual advertisements (or a combination), that are positioned on an already established website (with probably an established clientele) that has embedded information relevant to their products or services, so the advertisements are exposed to browsers who are potential customers looking for the same kind of information displayed by the host site.