Instagram is a free photo-sharing program and social network that was launched in October 2010. The service enables users to take a photo, apply a digital filter to it, and then share it with other Instagram users they are connected to on the social network as well as on a variety of social networking services.[3] As of September 2012, Instagram had 100 million registered users.[4] A distinctive feature is that it confines photos to a square shape, similar to Kodak Instamatic and Polaroid images, in contrast to the 4:3 aspect ratio typically used by mobile device cameras.
I'm using it and love it. It's sort of like Twitter, Facebook and stuff where you can follow people, but you have the ability to instanly post pics. Hence the name "Insta=Instant; Gram=pics." You can "Like it" and comment and type responses on people's pics.