It could mean a lot of different things, but because we are in the Internet Marketing field. Search Engine Marketing = SEM Search Engine Optimization = SEO
Yes. SEO = (Search Engine Optimization) and SEM = (Search Engine Marketing) SEM is more related to actually submitting your site to search engines and getting them to try to cache & index your page. Where as SEO is more about actually building and optimizing your site and content so that it ranks higher on the search engines.
SEO and SEM are the two common terms used by the webmasters. I think SEM is a broad term. It's everything that can be done to utilize the technology of search engines with the goal of promoting a web site and increasing its traffic. And SEO, therefore, would be a subset of SEM.
SEM is a much broader term than SEO, and actually includes SEO as one of its techniques. SEM is all that a company can do to advertise itself on a search engine, including paid inclusion and other ads.