This is a post which is motivated because of a question asked by @badfish2.
So here is the answer.
In the reader, you can have all the TAGS, like this. Here, in the reader we can see that all blogs from all the Tags are shown on the page. Once, you click any of the Tags you will only see blogs from that particular topic.
But, when you create a LIST you can group the lists into various headings/Lists like this.
I hope I was able to explain properly.