Search Engine Marketing is the practice of paid advertising to ensure your marketing content appears on SERPs. Marketers can bid on keywords that they believe consumers will enter into a search engine (such as Google) when interested in their products & services.
While SEO focuses on optimising a website to drive organic traffic and increase visibility, Search Engine Marketing puts more emphasis on paid advertising and can include more versatile versions of appearing on the results page, such as visuals of products at the top of the page.