It is common for many Twitter users to be tweeting about the same topics at the same time. This usually happens when major events occur and when there is a news about a celeb or public figure that is going viral. When those topics become so popular in discussions on twitter, they "trend" or become "trending topics".
Twitter then lists the current top 10 topics on your Twitter home and discover pages as "Trends List". Topics break into the Trends list when the volume of Tweets about that topic at a given moment dramatically increases. The Trends list captures the hottest emerging topics, not just what’s most popular.
According to a blog post by Twitter, sometimes a topic doesn’t break into the Trends list because its popularity isn’t as widespread as people believe. And, sometimes, popular terms don’t make the Trends list because the velocity of conversation isn’t increasing quickly enough, relative to the baseline level of conversation happening on an average day.
Trends are determined by a Twitter algorithm that identifies topics that are popular now, rather than topics that have been popular for a while or on a daily basis, to help you discover the hottest emerging topics of discussion on Twitter that matter most to you.
Some Trends have # sign before the word or phrase. This is called a hashtag and is included specifically in Tweets to mark them as relating to a topic, so that people can follow the conversation in search.
When signed into your account on the web, Trends will be tailored for you based on your location and who you follow. You can choose to Keep tailored trends based on your location or choose a nearby location or search locations.
Clicking any of the Trends takes you to the Twitter search results for that Trend. You'll see all Tweets including that phrase or hashtag. If you want to participate in a trend, just post a Tweet including the exact word or phrase as it appears in the Trends list (with the hashtag, if you see one). Due to the large number of users tweeting about these specific Trends, you may not always be able to find your particular Tweet in search, but your followers will always see your Tweets.
Recommended: How I Monitor Current Twitter Trending Topics