Search Engine Marketing (SEM) is paid advertising on Google’s search engine results page. This is also often called Cost-Per-Click Advertising (or CPC ads for short).
Search Engine Optimisation is the deliberate configuration of your websites text and structure to ensure Googlebot understands your content and can match users searching Google with specific content on your website. This flow of visitors to your website is free and is often called “organic traffic” to hint at the idea that this traffic is natural.