Server and client-side
When you write in an URL and hit enter in the address bar of your web browser, a request is sent to a sever where the site is located. The server returns a response to your browser containing the content to be shown by the browser as illustrated in the figure below.
The server typically also communicates with a database server. For instance, PHP code executed on the server can be used to fetch data from a database. When a client request a file from the server, the PHP-code on the server is executed, fetching the data from a database, and the result is returned to the client as seen in the figure below.