How do you use the tag system?

I'm assuming the tag system is set up so you can find a certain linked subject quickly??When the Gurus or Mods tag some threads/posts do you have guide lines on what to tag or with what name you give it?

I was trying to search on how to update NIS2007 as I know there are a lot of posts on that subject but I found no reference for it in the tag system and to find it in the search pane meant reading a lot of unnecessary posts.Is there a way of defining what is important to tag and what is not?

Example NIS2007 uninstall,NIS2009 install,

Is my interpretation of what the tagging system is for correct? Thanks.