Bike sharing is a service you can use that lets you rent a bicycle in your city. They typically have "docks" from which you retrieve and return said bicycle.
Citi Bike is the bike sharing service of New York City.
Here's a photo I recently took of a Citi Bike docking station near my apartment.
Now that we're on the same page about Citi Bike, check out an excerpt from this article in The New Yorker about two people who are trying to say they visited all of the 1,600 Citi Bike stations in the city:
Citi Bike had recently introduced a map denoting which docks a user had and had not visited: an invitation, basically, for the George Mallorys (Q. Why climb the mountain? A. Because it’s there) of socialized self-propulsion. Partway through his quest, he heard distressing news. A rival had been zigzagging across the city, checking off the same stations and surmounting the same obstacles—hills, dying cell-phone batteries, malfunctioning docks that nearly cost each the twelve-hundred-dollar lost-bike fee. In the end, Ambinder was beaten to the finish. He was bummed. When he completed his own journey, he thought, I gotta meet this guy.
Highly entertaining read. Do it.