This is called a QR code. If you have a smartphone, like a Droid, iPhone, and some others, then you can use the built barcode scanner apps (or download a free one if you don’t have a built in one) to scan the QR image. The image contains information which your phone can identify. In this case it’s a calendar event.
I should also point out that you can get the same information by going to http://usgs.gov/socialmedia and clicking the same image in the top right corner of the page…or you can go directly here: http://www.usgs.gov/socialmedia/qr/fb/11_29_2010.asp.