How to add the category of featured post and popular post?
not sure there is a way to do popular posts as it is a static site.
As for featured, you could add a front matter variable -
featured: true and then loop thru that to dispaly featured posts. Probably lots of ways to do it (catgories, tags, front matter).
What is the difference between the categories and tags?
not much really, as far as I know they have the same behavior, just an extra way to categorize things. For me a category would be something like Photoshop, and a tag would be layers, or tools, or actions.
So, how we add tags to a blog posts and and we make them show.