What is the difference between Search Engine Marketing (SEM) and Search Engine Optimization (SEO) in marketing?

Search Engine Optimization (SEO) is a component of Search Engine Marketing (SEM).

SEO is simply getting your site to appear highly ranked in search engine results -- without paying for it.  SEO will target different types of search such as images, geographic, text and keywords.  Once you have indentifed the keywords your customers likely will search for, you should build content relevant to your customers -- this helps raise the ranking in a search engine.

SEM also leverages paid techniques such as buying Adwords which force your site into the top of searches for those words - for a cost.