Originally published at: http://www.sitepoint.com/the-wordpress-template-hierarchy/
WordPress themes allow you to change the design of your website, providing control over the look and feel. They are essentially collections of different template files that work together and produce the overall design of your website.
Before you dive deeper into WordPress themes, it is important that you have an understanding of how WordPress operates and how the different pages of your website are created. WordPress interacts with a database to store your content. That means whenever you create a new post or page, the content of your post or page lives in a database and it’s only displayed using templates when the visitor asks for that particular content.
A WordPress theme is a collection of templates (CSS and PHP files) that WordPress uses to display the database content. WordPress will look for files in a theme to use to render the current page in a specific order. This is called a ‘template hierarchy’, that is used to create and extend WordPress themes.
WordPress themes decide how your content should be displayed on your website. A template in WordPress is a single file that determines what a specific page or set of pages look like on the front end of your website. When you change your theme, you are changing the way the content is displayed without changing the content itself.
WordPress themes use different templates files to generate a complete web page. For example, when you access a WordPress website, you can actually see multiple different template files combined together to create one complete page.
For instance, if you will look at an index page, like the front page of a blog, you will usually see a header created by the header.php template, content created by index.php, sidebar created by sidebar.php and the footer created by footer.php