What are the main differences between SEM and SEO?

Search Engine Marketing (SEM) is a broader term than search engine optimization (SEO). Organic Search Engine Optimization is when you do things to make your site appear higher in organic search listings. This involves keyword research, site structure optimization, backlink building, content creation, and link building. You can also do paid search engine optimization which is having your ad appear at the top or right-hand side of search results on Google, Yahoo or Bing. This involves choosing keywords and paying the search engines to display your ad on top of search results when someone searches for those keywords.

Search engine marketing (SEM) is the promotion of websites or web pages via paid advertisements in search engine results pages (SERPs). Search engine optimization (SEO) is the process of affecting the online visibility of a website or a web page in a web search engine's unpaid results—often referred to as "natural", "organic", or "earned" results.