What is this device that the person is holding as shown this picture, and what is the purpose of such device?
That is a set of 15 GoPro Hero 3 camera/camcorders on a custom mount.
One of my favorite examples of this in practice can be found in the DC Shoes advertisement videos with Ken Block, who is a professionally rally car driver. Take a look at the usage here in this YouTube video at 06:26, and again at 06:27 - http://youtu.be/btViXvIDsi0?t=6m25s
Here is a further example showing the mount and more details: http://mashable.com/2013/03/17/gopro-bullet-time/
Of course this mount can be used for both videography and photography, but most often I have seen it used in videography. The GoPro Hero 3 is much better for photography then past models, but it still isn't even close in quality to a DSLR.
For more information about GoPros being used for photography see this previous question: How does the GoPro camera perform for photography?