A word, phrase or topic that is tagged at a greater rate than other tags is said to be a trending topic. Trending topics become popular either through a concerted effort by users or because of an event that prompts people to talk about one specific topic. These topics help Twitter and their users to understand what is happening in the world.
Trending topics are sometimes the result of concerted efforts by fans of certain celebrities or cultural phenomena, particularly Lady Gaga (known as Monsters), Justin Bieber (Beliebers) and fans of the Twilight (Twihards) and Harry Potter (Potterheads) novels. Twitter have altered the trend algorithm in the past to prevent manipulation of this type.
Twitter's March 30, 2010 blog post announced that the hottest Twitter trending topics will scroll across the Twitter homepage. Users will also be able to find out how a specific topic became a trending topic.
There have been controversies surrounding Twitter trending topics: Twitter has censored hashtags that other users found offensive. Twitter censored the #Thatsafrican and the #thingsdarkiessay hashtags after users complained that they found the hashtags offensive.