You can take a look through the Calibre Manual for info on using the tag handling functions. Here is the search link:
http://manual.calibre-ebook.com/sear...s&area=default
Generally tags can be any word, term or phrase that makes sense to you. I don't recommend the use of the comma (,) inside a tag simply because that is the tag separator. Otherwise you might have "this, is, all, one, tag" which would be confusing - particularly if you add any of those words individually as tags (believe me, it happens).
Consistency is probably one of the more critical needs. Pick a system or style and keep to it. This becomes mandatory if you're downloading metadata. Otherwise you end up with multiple tags that are variants of the same concept, but have alternate spelling/layout (Fiction: Scifi, Fiction - Sci Fi, Science Fiction, Fiction (Science Fiction), etc).