Someone said on the hackers forum: “The analogy I use is that Categories are like a book’s table of contents, and Tags like its index.”

However, be aware that categories that are actual search terms are very strong Google magnets.

So, instead of using a category “lines we carry” think about “High Performance Motorcycles in Dayton OH”  and then use tags to highlight the brands and product names.

Tags are ways to identify the keywords or concepts in each post- categories group the posts in ways that make sense for others. Lately, Google has started dinging sites for putting a single post in more than three categories (which is too bad)- so think carefully about your choice of categories.