One of the many new words I've seen lately is "photobomb." What does it mean, and why?

Somebody unexpectedly intruding into your photograph, whether accidentally or on purpose. The someone need not be human. http://www.urbandictionary.com...e.php?term=photobomb

As you might expect, there's at least one website dedicated to photobombs: http://photobomb.com/

My wife keeps photbombing my pictures.

One of the funniest home videos I've ever seen was when my nephew set up his video camera so that he could capture his dive in their pool. It was hilarious because it built up to this dive - climbing up on the board, etc., etc., etc. Then, just as he dove, the big dog walked past and blocked everything - what a photobomb! The video used to be on YouTube, but I couldn't find it.