Hashtags (#hashtags) are a way to label, collocate and provide meta-commentary for online communication.
Closely related to the idea of tagging in general they were originally conceived of in 2007 by Chris Messina. Hashtags have primarily spread through Twitter where with the 140 character limit, space for organization is at a premium. Using the # symbol smashed into the tags without spaces makes hashtags uniquely searchable.
Since Twitter integrated their function to make them clickable they’ve become an excellent way to bring together tweets from many different users about a subject. By 2011 many conferences (including ALA’s events) have quasi-official backchannels set up through hashtags, like #ala11. Through the use of this ungrammatical tagging (with its roots in irc channels) a person doesn’t have to follow everyone at a conference to get an idea of what’s going on. A saved search for a hashtag covers much more ground, and can be easily abandoned when the event is over.
Hashtags are also good for more ad-hoc tweetable events like #libchat, as well as a means to participate in memetic trends (such as Charlie Sheen’s #winning earlier in 2011). Being a relative of leetspeak the use of hashtags can also be an important internet in-group signifier.