Search engine marketing (SEM) refers to the short text advertisements which generally appear at the top or down the right hand side of search engine results. These are often labelled as "sponsored Links" or "sponsored results". These advertisements are generally purchased from GoogleTM (the Google AdwordsTM product), or Yahoo! Search Marketing
Search Engine Optimisation (SEO) is defined as the process of building pages in a manner which enables the pages to rank highly in 'algorithmically' determined search results, which are generally displayed on the left hand side of the page. The 'algorithmic' results are independently determined by the search engine's own mathematical algorithm, and are based on relevance to the search phrase. Search engines use complex mathematical algorithms to determine the relevance of indexed pages for a particular search phrase.