Tag -- you're it
When lots of people are tweeting about a particular subject, hashtags make it easier to follow the conversation. You tag your tweets with a hashtag by typing '#' followed by the subject, without spaces, for example: #digitalbritain. You use this when you want to tweet about a particular subject. Doing so will allow other users to find tweets related to that subject, regardless of whether they're following you or not. For example, tweeting #uksnow will add your tweet to all the other tweets written about our recent winter wonderland.
Because Twitter is so organic, there isn't usually a hard and fast 'right' hashtag, but, with bigger events, they tend to spread so quickly that people will adopt the most popular tag. Some subjects become memes, with users inventing witty hashtags.
You can search Twitter, but, as with most things on the site, the best way to get the most out of hashtags is to look elsewhere. Hashtags.org shows you hashtagged tweets, with a graph of usage.
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