WordPress automatically recognizes template files with certain standard names and uses them for certain types of web pages. For example, when a user clicks on the title of a blog post, WordPress knows that they want to view just that article on its own web page. The WordPress template hierarchy will use the single.php template file rather than index.php to generate the page — if your Theme has a single.php file.
Similarly, if the user clicks on a link for a particular category, WordPress will use the category.php template if it exists; if it doesn’t, it looks for archive.php, and if that template is also missing, WordPress will go ahead and use the main index.php template. You can even make special template files for specific categories (see Category Templates for more information).