When functions, procedures, calculations, or other processing methods are performed on a server, they are considered server-side. By contrast, client-side is on the user's end. A good example of a script that executes on the server-side is a shopping cart or a search engine.
Today, almost all websites and online applications utilize both server-side client-side processing.
Server-side processing is generally slower than client-side, so programmers prefer utilizing client-side programming whenever possible.