If you are considering customising WordPress, you may be interested to know that it can be configured to use custom Page Templates for different Pages. This means that several or all pages could have their own unique Page Templates and so appear completely different.
If you want to create a custom page template called MyTemplate, create a file called mytemplate.php. At the top of the file, put the following:
<?php
/*
Template Name: MyTemplate
*/
?>
Copy the content of another template (perhaps page.php or index.php) after the above five lines of code then you will only have to alter the HTML and PHP code, instead of creating it all from scratch. MyTemplate will now be available as a choice when you create or edit a new page. You can go ahead and customise this new template further without if affecting your default pages.
To select a template, there must be at least one custom Page Template available in the active theme. Once there is, toward the bottom of the Write Page administration panel (or on the sidebar, depending on which version of WordPress you are using) you will see there is a drop-down labeled Page Template. From there you can select which Template will be used when displaying this particular Page, and MyTemplate should now appear.
See Codex WordPress Creating your own Page Templates to find out how to create a custom Page Template.