Digital Marketing is the umbrella under which numerous other tactics also fall such as SEM, SEO, Social Media, and PPC for example.
Search Engine Marketing (SEM) is a term that looks at the presence that a business or entity has across search engines and how to improve it, expand upon it – and give visibility to it. SEM would usually incorporate SEO and PPC.
SEO (Search Engine Optimisation) is the process by which you refine your website, the content on it, and the relevancy of the content in relation to the products or services being offered. So that when search terms are used in search engine queries that are relevant to your product or services – your business can be listed as relevant for those queries. Essentially SEO aims to increase visitors to your site.
PPC means Pay-Per-Click: This is usually referring to a paid form of advertising online such as Google Adwords or Bing Advertising (Microsoft Advertising). This is a service offered by a third-party provider that enables you to create adverts across search, display, and networks.