A question that comes up in blogging circles from time to time is what the difference is between Comments and Tags. Most blogging software allows you to mark posts with both, and the distinction is often times not clear.
I like to think of it as like walking into a supermarket. Categories are the big signs over the aisles, and tags are the contents labels on the products. Thus: Categories = broad sweeps; Tags = details.
Even without the guidance of blogging experts, that is the approach I took!