I want to add something one person said that I'm still pondering, "If tags are just keywords, what is the value of keywords when SEARCH is just going to find the search words in the body of the document anyway? What's the value of pulling them out as keywords?"
I’m sure someone within Jive will have a more thorough technical description of this, but the way that Jive search was explained to us:
First, search is separated into separate “areas. These are title, document body, tags, attachments.
Second, the weight of items in terms of search relevance is balanced by percent of content. So a keyword in fields like title and tags generally have a higher value because the number of words in those fields are generally small, where a document body or an attachment is likely to have a large number of words.
The other reason tags are important is in tag clouds. “Keywords” only end up in tag clouds if they have been added to content as tags. And tag coulds can be very powerful in helping users find content they are looking for.
Tags and Keywords are different terms for the same thing. The idea is that users can tag content and those tags will make it easier for others to find that content by browsing and searching. Tag Clouds will gather all the tags being used and will highlight the most popular tags in the system. Tags also feed into search results and are weighted slightly higher than the text in the body of the content.
Here's a wikipedia article on tagging that should help explain in more detail if you are interested: http://en.wikipedia.org/wiki/Tag_%28metadata%29