What is the difference between SEO and SEM?

In simple terms, the difference between SEO and SEM is given below:
  • SEO - Search engine optimization (SEO) is improving website visibility in search engines, usually by free methods like following HTML standards and building content.
  • SEM - Search engine marketing (SEM) is a type of online marketing that focuses on search engines - this can include normal search results but also PPC/paid search placement.

Therefore, search engine optimization (SEO) is a part of search engine marketing (SEM).