Categories are shown at the top of the sidebar on the main page. The categories are presented in a heatmap. If a category name is big and red, it’s hot. Meaning there are many posts in this category. If the category name is blue and small, it’s not popular. When you click on a category, the posts in this category are shown, starting with the latest addition. If the category has any sub-categories, they will be shown in the sidebar.