SEM, or Search Engine Marketing, is a wider activity than just SEO. Search Engine Marketing actually can include SEO, but more commonly has Paid Ads as its primary focus.
Danny Sullivan, one of the most respected experts in Search Engine technology, made the term “Search Engine Marketing” popular in 2001. It covers the spectrum of activities involved in getting a website better visibility in the search engines.
SEM may include any or all of these actions: SEO, managing paid listings at the search engines, submitting sites to directories, and developing online marketing strategies for businesses, organizations, and individuals.
Search engines change every year. People have learned to improve their methods of searching on these engines. With all this change happening, it is wise to consider that – any tests you have done with search engines years ago, may no longer be valid. You may want to try small tests with SEM methods to see if there are ways to improve your business.