Search engine marketing vs. search engine optimization?
 

What is search engine marketing and how is it different from search engine optimization?

SEO = the organization and curation (ie optimization) if your online content. You're basically helping search engines find your stuff, and in the method it wants.

SEM = marketing, but using the tools afforded to you by the search engines. SEO and SEM really do not overlap, and have entirely different uses. SEM is a subset of a larger marketing plan (assuming you have one, which sadly many do not). It may play off SEO'd content, or might go in another direction entirely like paid search (aka Google AdWords) that entirely ignores your SEO activities.

Got it? :)

I see a lot of really bad definitions for what SEO and SEM is. Too many "expert" bloggers have little knowledge in the real world of marketing.

SEM is the superset of SEO. This means SEO falls under the SEM umbrella.
SEM is a way of marketing your website on search engines like Google and Bing.
You can do SEM of your website on a search engine by bringing your website to the top of Google organically and it is called SEO.
SEM is done by PPC along with SEO. PPC stands for Pay Per Click. Google shows your website to people ahead of organic rankings if you pay money to Google.

I hope this helps!