WordPress themes require several files to work like header.php, home.php, sidebar.php etc. The coding of the WordPress themes is split into various files. You might be using WordPress for long time but have you ever wondered how a WordPress theme works or What is the structure of a WordPress theme.
Joost de Valk has created a nice infographic which explains the anatomy of a WordPress theme. It simply explains how your blog works.
Have a look at the infographic: