The object is thought to be a "dead" comet (no more volatiles to create the comet tail). It passed by at about half a million kilometres,which is about 30% farther out than our moon. It's over half a km in diameter, so it would have created continent wide havoc, had it hit. It was only discovered about 3 weeks before the fly-by; were it to have been on a collision course, we wouldn't have had much recourse but to "duck and cover".
The first photo is from the NASA site.
The next photo is after I did a bit of processing in the open source image processing program known as Gimp. Basically, I just turned up the brightness and contrast of the picture, bringing out the similarity to a human skull even more than in the original. It is a fitting reminder from space, of the danger represented by near Earth objects colliding with our planet.
Oh, and here's a link to some Halloween Sci-Fi :)
Free on Amazon for the Halloween weekend.