Clubs are a social area where you can do certain challenges and interact with the community. As of early May 2019, the feature is being A/B tested against Leaderboards and might be phased out. Users are making petitions to save the feature.
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The top level tabs have roughly stayed the same while the visual style has been tweaked to be cleaner and the owl has been redrawn.
The Clubs feature is being A/B tested against the Leaderboards tab (and some users aren't happy about it: Petition to save Clubs)