a hash tag is simply a way for people to search for tweets that have a common topic and to begin a conversation. For example, if you search on #LOST (or #Lost or #lost, because it’s not case-sensitive), you’ll get a list of tweets related to the TV show. What you won’t get are tweets that say “I lost my wallet yesterday” because “lost” isn’t preceded by the hash tag.
Hashtags believed to have originated on Twitter but, interestingly enough, it is not a Twitter function. Some believe it began when the broken plane luckily landed in the Hudson River in early 2009, some Twitter user wrote a post and added #flight1549 to it. I have no idea who this person was, but somebody else would have read it and when he posted something about the incident, added #flight1549 to HIS tweet. For something like this, where tweets would have been flying fast and furiously, it wouldn’t have taken long for this hash tag to go viral and suddenly thousands of people posting about it would have added it to their tweets as well. Then, if you wanted info on the situation, you could do a search on “#flight1549” and see everything that people had written about it.
Take this ashtag for example: #worstjobeverhad. This Hash tag would compel many others to share the worst jobs they’ve ever had, thus contributing to a fun conversation. It can be used for specific searches or individual twitters that begin them for their followers.
Text from http://www.urbandictionary.com
Image from http://beautifultrouble.org/