SEO - Search Engine Optimisation focuses on improving the rank of the site in SERP on particular keywords. SEO activities drive organic traffic to your website. These activities take time to rank your website higher and generating relevant traffic who are searching for products and services (Pull Marketing).
SEM - Search Engine Marketing focuses on improving the presence of the brand on online platforms through advertisement. Advertisements include Display Ads, Text Ads to drive the relevant and potential audience to your website who formerly were not aware of your brand.