WordPress uses the query string to decide which template or set of templates should be used to display the page. The query string is information that is contained in the link to each part of your website. It comes after the initial question mark and may contain a number of parameters separated by ampersands.
Put simply, WordPress searches down through the template hierarchy until it finds a matching template file. To determine which template file to use, WordPress:
Matches every query string to a query type to decide which page is being requested (for example, a search page, a category page, etc);
Selects the template in the order determined by the template hierarchy;
Looks for template files with specific names in the current theme’s directory and uses the first matching template file as specified by the hierarchy.
With the exception of the basic index.php template file, you can choose whether you want to implement a particular template file or not.
If WordPress cannot find a template file with a matching name, it will skip to the next file in the hierarchy. If WordPress cannot find any matching template file, the theme’s index.php file will be used.