CMS = Content Management System
Many who are used to static HTML pages often have a natural confusion as to how a CMS template really works. They are more accustomed to functionality and form blending together. This page will explain what a CMS template is in general concept.
With a CMS or CMF (Content Management Framework as in the case of Drupal), the template and platform core are two totally separate things.
I’ll give an analogy of a car to illustrate this.
Cars have two MAJOR components. An engine, and the body. Notice how each are totally independent of the other.
For example, if your car was in a hail storm and dents the body like a golf ball, your engine will still start and run your car. In fact your engine could care less about the size or extent of the damage to the body, and will carry on as normal.
All the same, you can have the slickest looking paint job and car body in the world, but of course it won’t go anywhere with a shelled out engine.
The engine is the core code base of these platforms - i.e. the actual files (usually .php files). It is what drives the function of your website. The template is just like the car body. It is ONLY concerned with how the core looks. Because of this, making a car can be done with two people, one working on the engine and the other on the car's body - at the same time with no direct effect to the other. Then when it is time it all fits together.
Due to this, I often tell my clients: “Don’t let the current look of your site impede progress in content entry.” Even when I do most have a hard time overlooking this fact. Like the engine and the car body, these elements are not married, and when the template is ready it will prorogate that look to all the pages anyway.
Finally, because of this you can easily create or have multiple templates that you can activate with the click of a button to instantly change the look of your site. Some of my clients have done this with seasonal themes. So when Christmas comes around, they activate the ‘Christmas Template’, and wala, their sites looks festive.
Another quick example is to look at it like changing clothes. Your core, your body, has 2 arms, 2 legs, head and torso. You can go to your closet and very quickly and drastically change the way you look. Whether you look like James Bond in a $1,000 tuxedo, 3 day old pajamas, or a work out suite - in all cases your core never changes. You still have all your arms and legs. At the end of the day all you did was change how that core was presented to the rest of the world.
This is what a CMS template is and does.