Trending topics is what makes twitter very unique and different. Many users turn to twitter for breaking and real time news discussions. Trending topics is one way all are notified of latest happening in the world.
Trending topics basically are the most popular topics at a given point of time. Twitter homepage displays a list of ten most popular trends.
But news on twitter is not always true, there can be rumors like the RIP jyoti basu when he had not even died.
Trending topics on twitter are very important and everybody has their own list of topics which they want to trend . If a topic manages to get into trends then everything will change radically for things related to topic. Like in case of #Chetanblocks he gained followers.
Trending topics consist of a lot of spam as spam bots use trends to gain exposure.
Tweeting about trends can also increase your followers and give you exposure. Twitter is trying best to improve its trends and has added the local trends feature recently.