A "category" on critic.ly is simply a comparison of different similar products. Consumers often would like to know what is the "Best of" a particular category. So, for example, the category may be "Hiking Boots for Wet Weather". The products compared within that category would be all hiking boots from around the internet that are really good in wet weather conditions.
Some of the categories on critic.ly come from the way products are compared and/or grouped together on other websites. However, certain categories are organically discovered by critic.ly when comparing and contrasting the data from millions of different products.