The next version of WordPress will have support for user-defined themes built in. Until now, WordPress used index.php and the comment files in conjunction with a CSS stylesheet to control the way it presented the content when published. All other pages, including the category and archive pages, were actually generated by passing parameters to the index.php page. The introduction of the theme system does not mean that the old method of styling pages will not work. The new theme system will provide a method for you to define different physical (.php) files for the different components of your weblog.
Link to share this: https://gabrielserafini.com/blog/2004/11/29/theme-development-codex/