People new to Twitter find hashtags among the most confusing aspects of the system. But it's one of the most useful conventions, and it's actually a simple idea, worth getting your head around.
Because there's no way on Twitter to categorize a message or to say, "All these messages are about the same event," users created an ad hoc solution: When somebody wants to designate related messages, they come up with a short term and prefix it with the # symbol. (In programmer-speak, that symbol is a hash mark, and the term is a tag; thus "hashtag.") Then others add the hashtag to messages about that topic.
Hashtags serve many purposes. Perhaps the most common is denoting events, like Module Midwest Digital Conference (#module09, shown in this shot from Hashtags.org). But increasingly popular are tags that signify messages from a group of people, like #tcot, shown here, which stands for "top conservatives on Twitter." Twitter memes also show up often, like #FollowFriday, shown here and described in Chapter 4 (the #follow tag is a variation). The #fb tag here is interesting: it's part of the way you can cross-post messages to Facebook, described in Chapter 5. In Chapter 3, we offer ideas on using hashtags yourself.
To see messages categorized with a hashtag, head to Twitter search (http://search.twitter.com, detailed in Chapter 2) and run a query for your term. Hashtags.org (http://hashtags.org) also shows popular hashtags and some stats on their usage. Tagalus (http://tagal.us) and What the Trend (http://whatthetrend.com, described in Chapter 2) can help you figure out what a particular hashtag is about.
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