What is photobombing?

Its when somebody jumps into the background of what was supposed to be a nice picture, usually sporting a funny look on their face--thus ruining the photo :D. There are numerous other ways to photobomb a picture, pretty much, just do something ridiculous in the background.

Here's an example of a classic photobomb: http://photo-bombers.com/wp-content/uploads/2010/02/Photo-129.jpg