If you are setting up a Wordpress blog the chances are that you will be looking into modifying the default theme to be something a little more customised to your site. Theme development can be as complicated or as simple as you want, or are capable of doing.
Wordpress themes are stored in the folder wp-content/themes/, each theme being stored in it's own directory.
The basic theme must contain two basic files, the main control is done from a file called index.php and a file called styles.css, which is also needed to allow you to display the theme within the admin section of Wordpress. If you don't want to use the styles.css file then this is fine, but it should be present and contain the following lines.