A Herpderpington is an individual who enjoys photobombing people's photos by making the typical Herp Derp face (or equivalent). This can either be done as a joke to be funny, or done to be an asshole. Usually, Herpderpingtons act as a group, so the name comes from a play on words on a family name or last name.
It is also the official title for those who herp derp.
Some of my friends were photoboothing before class when suddenly the Herpderpingtons photobombed us.. but the picture ended up being really funny!