Are you referring to Wordpress or Joomla tags? Those are a way to group together pages/posts dealing with a similar topic. Just like categories. And just like categories, tag pages should be noindexed, because they are at best a set of links and at worst duplicate content.
In relation to keywords, are you referring to the keywords meta tag? This one is useless, Google ignores it but if it's extensive it may consider it an attempt at spam. In relation to search, keywords and keyphrases are any words or phrases which appear on pages.