Every WordPress site has an active theme: a standalone piece of software designed to integrate with WordPress's core software in numerous defined ways.
A WordPress theme's job is to control the site's presentation: how its data displays to user.
Presentation is a huge topic, encompassing layout, typography, responsive design, display and formatting of content, image sizing, accessibility, and more. This means that a theme doing "just" presentation is still a sophisticated work of web design and development.
It's possible to make themes do more than presentation—for example, register custom post types or sitewide data—but we call that practice "theme creep" and we advise against it.
You've used WordPress themes before. If you're anything like us, they're one of those WordPress features that made you sit down and take a deep breath when you realized how cool the...