Wikipedia has a lot to say about the word "hashtag," but (as it sometimes does) misses the actual news about this word:
It has been named the Word of the Year by the American Dialect Society.
At the hashtag.org website, you can also read the real lowdown on this word, including how to start a Twitter hashtag and make it popular ("trending").
Here are some "trending" hashtags: #Obama2012, #RomneyRyan2012, #Mitt2012, #tcot, #gop, #tlot, #teaparty.
Madame L suspects her Dear Readers know more about trending and hashtags and trending hashtags than she will ever know, but she just wanted to get out the news about this Word of the Year.