Re: The cool photos thread (Bandwidth Warning, Yo!)
*ahem*
A hashtag, often defined by a #, as in #hashtag, is used, primarily by Twitter and Instagram to gather information about trending topics. If thousands of people use a hashtag like #YOLO, Twitter and Instagram will gather that YOLO is now popular. The hashtags are also linked to your location, and enables both companies to have WWTT, or World Wide Trending Topics.
How is this useful for you? In no way whatsoever.