Why do people use # on Twitter?

[Senioritis]

When people tweet on Twitter, they sometimes put a # before a word or phrase, like #iranelection or #michaeljackson. What's the point of this?

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I think people just use it to make it a Trending Topic..

[basketball213262]

to markk it to a topic.
like on the right hand side of your twitter homepage where it says Trending Topics, people put the # so that if someone searches the topic up, the person's updates come up

ex: R.I.P Michael Jackson #Michael Jackson

if I were to search up Michael Jackson, this person's update would pop up.
Hope this helps