With the use of one symbol, netizens can link with like-minded souls over a particular subject whether it's music, a product, a breaking news story or even just a mood.
The humble # has leapt from its lowly status of sharing space with the number 3 on your keyboard to dominate conversations and messages across the Internet.
For some people, it is even part of verbal conversations.
The hash key (sometimes called the "hex key" here or "pound sign" in the US) was earlier used to indicate an object's number or edition.
But that changed in August 2007. Mr Chris Messina, an open-source advocate in the US, posted on Twitter proposing the four-stroke symbol be used to indicate groups on the microblogging service.
"how do you feel about using # (pound) for groups. As in #barcamp [msg]?"
Using the symbol meant that all discussions and messages about Barcamp (an online conference) could be easily grouped together - essentially, adding that symbol created an open chatroom.
Twitter originally rejected the idea until October of that year when citizen journalists used #SanDiegoFire to tweet about forest fires in that area.
In early 2011, hashtags helped highlight events in the Middle East as social media was cited as part of the success of the Arab Spring movement, which saw uprisings against various regimes in the region.
In the first three months of that year, "#Egypt" had around 1.4 million mentions, "#Libya" and "#Bahrain" had over had 990,000 and 640,000, respectively, and "#protest" had over 620,000.
Hashtags are a way of gaining attention.
Social media scientist Dan Zarella claims that tweets with hashtags are 55 per cent more likely to be retweeted.
For individuals using hashtags in their tweets, engagement can increase as much as 100 per cent, while brands could achieve a 50 per cent increase.
There are the obvious ways - which is to just add them to the end - if you are talking about a particular subject.
Hashtags can also be used to make a particular statement (#fail or #YOLO).
Sometimes they are there only to emphasise a statement.
Less of a group message and more of a punchline (#peanutbutterrocksmyworld) though it would not be unheard of for an obscure hashtag to take off.
Some platforms, such as Instagram, have no character limits and can see posts accompanied by a wave of hashtags looking to grab attention.
There have been some high profile #fails in the use of hashtags.
Something that singer Susan Boyle's PR team could/should have been more careful with while launching her debut album.
The phrase Susan Album Party was turned into the hashtag #susanalbumparty.
Many corporations, McDonald's, Qantas and US chain store Walgreen's have had their hashtags backfire.
Trying to drum up positive publicity about their service, with #McDStories, #ILoveWalgreens and #quantasluxury, Twitter users added the hashtags to complain or poke fun at the businesses.
Some people have also objected to the phrase hashtag being used in actual conversations. "I want to get some chocolate, hashtag, rules my life."
This use has even been parodied by US talkshow host Jimmy Fallon who shows how annoying it is in a skit with Justin Timberlake. On YouTube it has close to 18 million views.
#YOLO: You Only Live Once. A modern day carpe diem (or seize the day). Can be used straight. "Going skydiving today #YOLO" or ironically "Cabbage soup for lunch #YOLO".
#FOMO: Fear Of Missing Out. The idea that not doing something will mean missing a life-changing experience. "Not staying in tonight because #FOMO".
#TBT or #throwbackthursday: Usage depends on character limit. A recent development that highlights nostalgia. "Remembering beach holiday last year because stuck in office now #tbt".
#FF or #followfriday: A benevolent act where each Friday, you highlight to your followers who is also worth following.
#nowplaying: Essentially what you are listening to.
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