Problem with category template

Per this article (http://premium.wpmudev.org/blog/cool-wordpress-category-pages/), I am attempting to change the format of my blog category template so I can add some introductory text to the beginning.

I have a description box on my Edit Category page, but when I add text to that it doesn’t show on the category page. I thought I would check the category template of my theme, and noticed that I don’t have one.

Where is the styling for the category page coming from then? Any tips on what I can do?

Thanks in advance.