In Nucleus, both templates and skins are used to determine the way your blog looks. So, what is the difference between these two?

1. Skins define how your sites look. Each skin consists of several types: one for the main index, one for the detailed item pages, one for the archive, ... The skins also contain instructions of where to include a weblog, and which template should be used to do so.
2. Ha! This means templates are used to define the way the weblog block in your page looks like. The reason why templates aren't included in the skins themselves, is that several skins can use the same template to display a blog.

An example is given in the image below. The whole page is defined by a skin, while the parts in the red rectangles (category list and blog contents) are formatted according to the templates. It's the skin that defines where the red rectangles will appear.