Search engine marketing (SEM) is a form of Internet marketing that involves the promotion of websites by increasing their visibility in search engine results pages (SERPs) through the use of paid placement, contextual advertising, and paid inclusion. Depending on the context, While SEO is Search Engine Optimization.It includes the techniqes of offpage and onpage SEO.
SEO is the act of optimizing a website for organic or natural search engine listings, where as SEM is the process of marketing a website via Internet search engines by improving rank in organic listings, paid or sponsored listings or a combination thereof.
So you can say,
SEO is a component of SEM.