SEO - Search Engine Optimisation focuses on improving the rank of your website in SERP (Search engine results pages) on particular keywords. SEO activities drive organic traffic to your website. SEO activities take time to rank your website higher on Google or any search engine and generating relevant traffic who are searching for products and services.
SEM - Search Engine Marketing also called PPC (Pay Per Click), concentrates on improving the presence of the business or brand on online platforms through advertisement. Advertisements include Display Ads, Text Ads to drive the potential audience to your website who formerly were not aware of your brand.