Difference between SEO and SEM
SEO is sometimes used as an umbrella term for SEM, but because SEM only refers to paid advertising, they are distinct. SEM is concerned with acquiring traffic through paid advertisements, whereas SEO is concerned with acquiring, monitoring, and analysing organic (unpaid) traffic patterns.
Keywords are heavily used in SEO and SEM to drive traffic to business websites and web pages. However the marketing techniques employed in SEO vs SEM are distinct. They are both concerned with traffic flows and how they relate to marketing activities. People will use search engines to find what they're looking for, and they'll be able to find it through organic (SEO) or paid results (SEM).