As I understand the term, "taxonomy" is just a way to group your posts, like "products." You can indicate your post is a "product" by specifying "product" as a taxonomy of the post in wp_insert_post(). But you could also just add a custom field to the post, like "group=product" and then to get all the product posts, ask for all the records that had a custom field of group whose value was product. I'm assuming, though, that the latter method would require WordPress to look through all the posts for those that had a custom field of group whose value was product, while the taxonomy method essentially threads all the records together that have the taxonomy product so they're accessed directly instead of through a search through every post. Is that correct?
Second, is "category" a built in taxonomy? So that if I assign a category of product when I do my wp_insert_post() then when I ask for all records with category =product, will WordPress be able to access all the product posts directly, i.e., all the posts with category=product?
Thanks for your help.