What is a Twitter Hash tag?

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I have been using twitter for a couple of months now. But I am confused by the use of hash tags. I have searched for every explanation possible but I just don't get it. Can someone please explain it to me the simplest form?
Ok, so you can make up your own hashtag?

[Erica Nelson]

A twitter hashtag is the "#" symbol. You put it in front of a short phrase and it is used to group similar tweets. Some phrases may be, "#ThisSummer... then write something," "#WhyBeInARelationshipIf ..then write something"

Now, once you tweet that, the #ShortPhrase will become a link. When someone clicks it they will be relocated to another page that has a list of tweets that use that same hashtag.

Also, if you ever noticed on the right hand side of your homepage you see "Trending Topics". These are topics that many people are tweeting about, and a lot of the time you see hashtags. It's fun to participate and use that same hashtag.