If you’re new to Twitter you’ve learned replying, sending direct messages, retweeting and finally got to a stage where you don’t know why people are adding hashtags (#) or pound symbol at the end of their messages like #followfriday #Ladygaga, you don’t know what is that hashtag meaning either.
Twitter hasn’t provided users to any way to group tweets, people created their own hashtags to categorize their tweets. Everyone can create a hashtag for relevant topic but not for spamming purposes.
People add hashtags at the end of tweets, most of these hashtags are indexed by Twitter and shows them as trending topics. Whenever you search for a particular hashtag your hash tagged tweet may appear in twitter search results. We can call trends as breaking news appears on homepage of twitter.
Hashtag.org shows popular hashtags used on Twitter.
If you wanted to know about particular hashtag visit what the hashtag, enter that hashtag in the search box and this service shows hashtag definitions, resources and usage statistics and related hashtags also.
For example I searched for #fb hashtag, see the information provided by this site in the screenshot for that hashtag.
Tagal.us similar kind of site to wthashtag.com shows briefly definition of the tag .