Technorati explains tags as being pretty much the same as categories. If you are a blogger tagging your posts, tags are not categories. I don't want to have a gigantic list of categories. I want a realatively short list of major topics. In order to assign a category to a post, I have to add it as a category on my category list. So, I'm limited and don't assign a new category to an individual post unless I really think I'll use it often.
Tags are Different. There's no tag list - at least not yet*. When I tag my posts, I can use anything on a one-time only basis and not worry about it. I can tag for SXSW even though it's not an on-going category. Big difference. [*If there's going to be a tag list, the infrequently used tags will be tiny in my tag list and the frequently used ones will be big, so more tags can be in the list without making it unwieldy.]
Tag Language. Because this is folksonomy territory, though, speaking in the language of tags, you may want to use words that others and their software/services understand. I may want to keep a copy of Technorati's tag page handy so I can see whether to tag this as blog, blogs or blogging or tags or tagging or all of these. Since this is a bottom-up thing, I get to decide whether to (a) be casual and just tag the first thing that comes to mind or (b) go with the majority so I can affiliate my post with more posts or (3) think about it and then put the best tag on it that I can think of, not considering anyone else's nomenclature or (d) *vote* for the tag I would like to add juice to. It's very fun and interesting.
Tag Tip #1. If you put your tags into your post at the beginning, you can jump to those Technorati tags for more related info by clicking on them in your post preview.
Tag Tip #2. I like to illustrate my posts and decorate my post/feed with a graphic for each post. Technorati tag-surfing may help you quickly locate that little graphic for your post. That's how I got the graphic for this post which I copped from a Flickr photo of Peter Morville's great slide.
By the way, I much prefer to newsread, browse, search or tag-surf blogs than the bigger webspace. Googling floods me with low signal-to-noise ratio info-stuff. Sorting through all that feels too time-consuming now that I have better alternatives. Sometimes I need to search in that big noisy space, but not all that often. With that in mind, I've added the Technorati Searchlet to this blog page.
Technorati Tags: tagging, social software, technorati, technorati tags, folksonomy, blogging