Tags are ways to categorize topics with a finer grain than the general forum categories us admins have to use when creating forum themes.
For example, there are no 3 prong flash hider forums on aktalk.com. There are places you can talk about 3 prong flash hiders, but not a forum just for that purpose.
What if you wanted to see all the topics talking about 3 prong flash hiders?
In the past you would have to do a search, but what if someone used the term "three prong" instead of "3 prong"? You might have to come up with a creative search term to make sure you got all the topics you're interested in. Even then you'd still have junk you didn't want to see and possibly be missing the very topic and posts you need to see for your research.
But now what if you wanted an RSS feed of all 3 prong flash hider topics?
Without tags, this would be impossible.
With tags all you have to do is go to http://aktalk.com/tags/3-prong to see all the topics that were tagged with "3-prong". What's even neater is that you can subscribe to that tag's RSS feed and monitor all new 3 prong flash hider topics.
What if the post author doesn't use tags or doesn't include all the ones they should?
Not a problem! Anyone can add tags to any topic at any time. This ensures that topic tags are kept up to date with each new post.
How do you visit a specific tag's page?
Just type it in! http://aktalk.com/tags/<tag name>
For instance, single point slings would be http://aktalk.com/tags/single-point
You can also find links to all the most used tags on the homepage at http://aktalk.com
Tags also help search engines find your post easier, which can be important for those Market posts where you want to trade up or sell your old accessories!