Every year, the analysts and engineers at tweeter crunch all the tweet data for that year and present interesting trends. They then compile this wonderfully interesting twitterified data into a special site where anyone can go and explore.

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They have broken down their findings into these categories (with the twitter description for each)

• Golden Tweets: The top two Tweets that generated the most Retweets for the year, plus honorable mentions for a few others that caught attention around the world. 
• The pulse of the planet: Some of the biggest conversations of the year that generated large numbers of Tweets and Retweets. 
• Only on Twitter: Moments of serendipity and just plain awesomeness (if there is such a thing) are what we call #onlyontwitter. We list examples of Tweets that came to life organically on Twitter, and can be appreciated in an instant. 
• Top trends: Lists of the significant Trends of the year in the U.S., UK and Japan. These phrases reveal what captured our attention based on sudden spike of interest, rather than what was consistently popular. 
• New voices: From the Pope to Pelé, this year Twitter welcomed new voices from every corner of the world.