I've been visiting various sites related to blogging in general (as opposes to dedicated WordPress sites), and often see tags and labels mentioned. For example, http://www.thegoodblogs.com/ talks about tags to categorise a blog, other blogs list "labels", which seem to list only posts with that label.
Are categories, tags and labels all the same thing? As WP has categories already built in to its structure, do I need to have tags or labels? I see one site actually said that WP cats override the need for tags - but is that in all sites?