Nucleus is a base theme for Drupal (version 7+) developed by ThemeBrain. It aims at facilitating the process of coding a given design into a Drupal theme. It is a powerful tool for Drupal developers and themers, as you can create layouts and styles within several clicks without having to code it from scratch.
As you get familiar with Nucleus, your theme project will get finished in a fraction of the usual time needed for the same design, thus making you more competitive and focus on more important stuff.