Key Takeaways:

WordPress lives on a web server, and delivers web content to a user's web client (his or her browser).
Processes that occur on the server are called "server-side," as is the server environment itself.

Processes that occur in the web client (browser), and the general environment of the user's browser and device, are called "client-side."
Key server-side elements in WordPress include: PHP scripts, which run only on servers and are a WordPress site's main ingredient; and the WordPress database, which stores all WordPress posts, post data, and user data (among other things), and which exchanges data with the server's PHP scripts via a special language called SQL.
Key client-side elements in WordPress include: HTML markup, images and other media files, and CSS styles, which the server delivers to the client to be rendered into a human-viewable webpage; as well as Java...