You can add description for each of your category in Ecwid control panel → Catalog → Categories. Select the category what you want to add description just clicking on it in the list. After that category info like name, image, status and description will appear in the right part of the page.
You may notice that it is not just text area, but a WYSIWYG editor, which allows you not only add the common text, but also format it in the convenient way like in the text editor.
Using it you can make paragraphs, add images, change color, size and font of the text etc. In other words you don't need to know HTML for creating description with styles you want to.
If you want to set category description using HTML tags you just need to click small "HTML" button in top right corner of WISIWIG editor and pop-up window with source HTML code will appear. The code in this window can be changed according to your needs.
This feature is extremely useful if you want to make your category pages more appealing, which can lead to more sales. For example you can add a general description of a categories products or a graphic banner, promoting a special product or highlight some featured products (two products below the “This Weeks Featured Specials” header are added using a category description). Using this feature your category pages will look better than ever.