What is the difference between SEO and SEM?

Search Engine Optimization is “the process of maximizing the number of visitors to a particular website by ensuring that the site appears high on the list of results returned by a search engine.” SEO is comprised of two different activities
1.  On-Page SEO
2.  Off-Page SEO
“Search engine marketing is a form of Internet marketing that involved the promotion of websites by increasing their visibility in search engine results pages (SERPs) through optimization and advertising.” SEM includes SEO tactics, as well as several other paid search marketing tactics such as Pay Per Click (PPC) listings and advertisements.
The main difference between these two terms is that SEO is simply a component of SEM. As mentioned above, SEM includes various components of paid search, such as PPC and also SMM (social media marketing).