Wordpress theme is merely a folder which is to be placed in wp-content folder of wordpress hierarchy .
The basic files needed in that theme folder is index.php and style.css
In this style.css the following lines are very essential.
/*
Theme Name: Theme Namet
Theme URI: http://www.sajithmr.com/
Description: Details of your theme
Author: Author name

Author URI: http://www.sajithmr.com/
This theme was designed and built [...]