What is SEO?
SEO = Search Engine Optimization, ie getting your site ranked higher so more people show up at your doorstep.
In theory we’re interested in all search engines. In practice SEO = Google.
What's the difference between SEO and SEM?
While some people use SEO and SEM interchangeably, SEO (search engine optimization) is actually a part of SEM (search engine marketing).
SEO refers to the process of using on and off page factors (typically free) to get your web pages ranked for your chosen keywords in order to get more search engine traffic to your sites. SEM takes it a step farther to include using paid search engine listings and paid inclusion to get more traffic to your websites.