Twitter help! Will give BEST answer!?

[Mahadewa]

Trending topics in Twitter are Topics (basically words) that are currently mentioned 'a lot' in Twitter Public Timeline.
How does a topic become a trend ? Well, simple, because a lot of people are twittering about it (mentioning the word in their status update).
The # symbol is called Hashtag. A hashtag is a keyword or keywords which is prefixed by a hash symbol (#), and it is usually used to mark a tweet (a status update), to tag it, that it belongs to a category or more.
It doesn't have anything to do with Trending Topics, it's just that most topics that people tweet about are using Hashtag.