I'll try to further elaborate

The stories (or what most nukers refer to as News) is for posting things that change alot. For instance you may have many new news stories everyday. But these news stories can be catagorized into topics so that if people wanted to later search for an article they read on your site they can just look at certain topics. Also if someone reads one story that interested them there may be other stories that are of interest to them in that same topic.

Where as the content pages were made to display content for your site that is pretty static and doesn't change daily. They are pretty much like html pages are on a normal site.

So thats why they have two different "content" type systems. And each have thier own way of catagotizing them. But on many peoples sites that catagories might not be the same as the topics they use in thier news postings so that is why one uses catagories and one uses topics.

Does that make anything clearer?

Lastly if you are new to nuke i would highly reccomend going to www.nukecops.com and read the Nuke for Idiots Guide (Don't get mad I'm not saying you are an idiot thats just what they have named it ) It really helps explain alot of the basic nuke concepts.

Good Luck!