Categories are great for segmenting your content and will also allow readers to filter your content based on the categories you created. Articles/posts can be assigned to multiple categories.
Adding a Blog Article/Post to Categories:
To add a blog article/post into categories, go into the post settings menu and click the '+' button in the 'Categories' menu. Then type or select your categories and save.
How To Display Your Blog Categories:
You can display the categories in your sidebar (or anywhere else on your site) by using the Blog widget. Here's how...
Drag the Blog Widget into where you want it on your page.
Select 'Most Popular Categories' from the drop-down menu in the widget settings.