Is there a standard way of using Categories, Keywords and Tags in the context of blogs, websites and web applications? I am not sure if it is a case of these being equivalent of synonyms in some cases, but I don't recall seeing keywords and tags used together. Also, in terms of usage, it is not common to see the word categories used because these are normally specific to the type of blog or website, whereas keywords and tags seem to be used in a more generic sense. I have also seen the word Label used in a similar way.
I would like to know whether these terms have specific meaning or relate to specific parts of the website content and user interface design. It seems like these terms are concepts for physical and logical grouping of content.
Some suggestions are that there can be fixed (categories) versus user generated (tags), or broad groupings (categories, labels) versus specific groupings (keywords, tags).