Readers of this newsletter and my Gearhead column will be well aware that I am a big fan of Twitter the microblogging engine - a mechanism for posting messages up to 140 characters each. Your messages can be part of the public message stream or restricted to only your friends. As a Twitter user you can "follow" other users. This means that you see their "tweets" (the term for a Twitter message). They may or not follow you.

This may appear to be trivial, but the service has attracted millions of subscribers and become a major online phenomenon. One of the things that intrigues me about Twitter, in addition to how many people are very committed and enthusiastic about using the service, but the incredible creativity that has been applied to a core Twitter functionality such as clients, as well as add-on services.

An add-on service that may be new to you is something called “hashtags.” Hashtags are simply any string following a “#” sign, for example, #webapps. The Hashtag service (identified as the Twitter user “hashtags”) identifies who are its followers and follows them in return thereby enabling it to read all messages that may include hashtags.

So when a Twitter user who follows the user #hashtags posts a tweet that includes one or more hashtags the Hashtag service indexes the tweet. This allows the tweet and all other tweets that use the same hashtag to be found using the Hashtag service.

As an example of how hashtags are used in practice, for the recent MacWorld conference the hashtag #MacWorld was defined. One of the tweets on June 15 posted that you could find, using the Hashtag service and searching for #MacWorld, them “Getting ready to watch the #MacWorld keynote” through the specially created MacWorld site or through the regular hashtags index.

Where hashtags can be really useful was shown during the wild fire season on California last year. The San Diego fires were particularly bad and the hashtag #sandiegofires started to be used. For those interested in Twitter updates (which were occasionally more useful than “official” updates) using the Twitter command “track #sandiegofires” sent all of the relevant tweets to them by SMS or instant messaging (Twitter’s track command doesn’t work with the Twitter Web interface or the Twitter API).

Mark Gibbs is a consultant, author, journalist, columnist and blogger.