The obvious definition of a hashtag is a word or combination of words, preceded by the hash sign (#) more commonly known as the pound sign on your phone.
The practical definition is that hashtags are identifiers used on microblogging sites such as twitter to link a post to a particular subject. This allows other users to search for a hashtag and see all of the posts related to that topic.
Hashtags are not native to Twitter – they are in fact borrowed from IRCs (i.e. Internet Relay Channels such as chat rooms). The Twitter community adopted them early on to allow for easier searches and filtering.
Twitter changed the game a bit in 2008 when they purchased the search engine Summize. The ability to search Twitter with keywords has meant that hashtags were not quite as necessary for trend tracking as they were in the past. Hashtags however still offer unique advantages. For one, tagging a tweet can allow it to get categorized within a topic that might not be mentioned in the tweet itself. This is often the case with quotes. It can also be useful when you are tweeting about a subject that is part of a bigger picture.
– Bill C-32 was just passed into law #copyright
Lastly, it can serve as a visual indicator to others following your Twitter stream that you are actively tweeting about a particular topic.
Speaking of topics, hashtags also show up in a number of trending topics websites, including Twitter’s own front page. One phenomenon specific to Twitter is the micro-meme. Micro-memes are emergent topics for which a hash tag is created, used widely for a few days, and then disappears.
As with any form of communication, there is etiquette to using hashtags. As a general rule, hashtags are placed at the very end of a tweet. This makes them easy to find and access. But there are exceptions. If done in moderation, it is accepted to insert a hashtag within a conversation, a bit like an embedded link, provided it is related to the tweet.
Example of a traditional use:
All my life I’ve looked at words as though I were seeing them for the first time. – Ernest Hemingway #quote #hemingway
Example of embedded use:
I am heading off to #Toronto to cover the #G20 meeting.
Here are a few commonly used hashtags for the Government of Canada:
#goc – for Government of Canada
#w2p – for the Web 2.0 Practitioners Group
#gov20 – For web 2.0 initiatives in government
#G20 and #G8 – can you guess what for?
#cpsr – for Canada Public Service Renewal
#GCpedia – for posts on GCpedia, the Government of Canada wiki
Cool. We added a list of gov’t related hashtags here..
Another great benefit of hashtags is to add context while being concise.
The hashtags are not active because that was a bit of laziness on my part. You can create a link to what is essentially a hashtag search on Twitter (as I have now done for #goc above) that will direct readers to it.
If you see a hashtag in a post on Twitter or in your Twitter client, it will usually be active and will lead you to the same results.
In terms of creating hashtags, you can have a look on http://www.hashtags.org/ to see if one already fits. If not, you can just create one.
Hope this answers your question.