Search Engine Optimization
the process of increasing the amount of visitors to a Web site by ranking high in the search results of a search engine. The higher a Web site ranks in the results of a search, the greater the chance that that site will be visited by a user. It is common practice for Internet users to not click through pages and pages of search results, so where a site ranks in a search is essential for directing more traffic toward the site.
Search Engine Marketing
SEM is often used to describe acts associated with researching, submitting and positioning a Web site within search engines to achieve maximum exposure of your Web site. SEM includes things such as search engine optimization, paid listings and other search-engine related services and functions that will increase exposure and traffic to your Web site.
Search Engine Submission
This is the actual process by which a site is directly submitted to a search engine for inclusion into their database. Some search engines charge a certain amount for 'guaranteed inclusion' into their search index.