As long as a website has one resource that is generated when it is requested by a HTTP client, it is considered to be dynamic. For example, using PHP to aggregate external CSS scripts for our website will turn our website into a dynamic one.
Given these points, a dynamic website is usually served by HTTP servers that are customised based on business logic.
Unlike a static website, a dynamic website is used for applications or content that are long-lived.
Most of the websites on the Internet are dynamic websites.
For example, Facebook's homepage returns different content for different users.
Similarly, Google's search results are different for different search queries. This is despite the fact that our browsers access the web page via the same URL.