Hashtags are keywords that are preceded by a pound symbol, allowing posts on supported platforms to be tagged.  They became popular on Twitter as a way of grouping together a large number of (publicly viewable) posts from authors with no other connection.

It allows for mass numbers of quick responses to a topic.  For instance, when there’s an earthquake in Los Angeles, dozens of posts will show up within second under #earthquake.

Some missteps have been made with hashtags, such as TV shows including live hashtagged tweets on TV without proper filtering, leading to a rush of offensive digital graffiti.

Twitter users converting to Facebook started using hashtags on their posts, despite the Facebook platform not supporting the hashtag system.  This lead to annoyed and amused Facebook users posting mocking and ironic hashtags.  The tables were turned when Facebook decided to support hashtags, retroactively enabling the ironic tags in old posts.