Search Engine Marketing is the effort to ensure that your website pages appear further up the listing in search engines.
Similar to search engine optimisation - that is effectively looking at each individual page.
Search engine marketing is looking at all the pages in the round.
Effectively a relatively academic distinction which in the cold commercial world is not of much practical help, spending time debating the difference. As a website owner your want your website to come up in relevant searches, and for sure you will not be interested in whether that is as a result of clever search engine marketing or skillful search engine optimisation.
Many successful sites rely on a battery of paid for advertising and success in organic listings combining the two can provide very valuable feedback as to what words customers are using to find your site.
Few businesses who seek to expand can afford to ignore these issues.
Sales growth typically relies upon being found in search engines and everything which contributes to that should be regarded as part of the marketing mix.