SEM, search engine marketing, is the practice of using several different methods of reaching potential website visitors via the search engines. It almost always uses SEO, which is the practice of optimizing a website to rank automatically and organically for free in the main body of search engine rankings. That's the common bond between SEM and SEO, but it's when SEM begins to build and expand on this base that things really start to get interesting.
SEM can also use Pay-Per-Click (PPC) advertising to bring in laser-targeted visitors-the type of visitors that can be hand-picked and completely ready to convert. And then there are other less common internet marketing strategies, like social media optimization, that can be worked into a comprehensive SEM plan. The bottom line is that by putting all of the most effective ways of using search engines to reach visitors together, SEM is a much more comprehensive, balanced approach when compared to just SEO alone.