In order to activate category descriptions, make sure that in Admin » Configuration, that Allow Category Descriptions is set to True.

Next, simply create your categories as you normally would by going to Admin » Catalog » Products/Categories. You can add a new category or edit an existing one.

When adding new or editing and existing category, you will see a page with several fields:

• Category Name: This will appear in the Categories Box in your catalog.
• Category Heading Title : This is the text that will replace the stock Let's See What We Have Here text.
• Category Description : This can be a detailed description of the category. You can use text or paste in HTML if you prefer. It accepts pretty much all simple HTML and will format correctly on your site.