Dave: What are categories? Hmmm, maybe I misuderstood keywords. What's the diff betw categories and keywords? Isn't keyword just a simple case of category? (Please don't yell at me.)

Since we have been working with topics and categories both to author ENT, an RSS namespace, and for our own topic based RSS aggregator, K-Collector, I have asked myself the same question many times: here's my 2 cents.

I have always thought about RSS categories in the same way as the term is used within Radio UserLand: a category is a channel I can route content to. This post could, for example, belog to the "technology" category.

Topics (or keywords or tags, I pretty much see them as synonyms) are more fine grained, they do not represent a category or a channel a post belongs to but describe the content of the post. For example this post could use topics such as "Dave Winer", "RSS", "Categories" etc.

Within ENT and K-Collector, topics are grouped in classifications and a topic cannot logically belong to two classifications. In K-Collector, for example, we are using as default classifications "People", "Things", "Places" (which we renamed Who, What, Where).