A disagreement on an Internet message board wherein one person says something wrong or offensive, and a large number of people comment in response to tell the person how wrong and/or horrible they are, and continue to disparage the original commenter beyond any reasonable time limit.
People commenting at the same time, without realizing others are jumping on the person as well, do not make a dogpile. The requirement is to join in with an angry group to yell at an easy target, or to get popularity points for being seen to agree with the group. They see that everyone else is doing something, and they copy it.
The original commenter typically does not respond at all, because they are completely overwhelmed or scared off. Once a dogpile has been established, an apology from the original commenter is less likely to be effective.
A dogpile in action
A 10:00 AM: "That song is retarded."
B 10:01 AM: "Um, that's really offensive."
C 10:01 AM: "That's not an okay thing to say."
D 10:02 AM: "Fuck you."
E-Z over the next 12 hours: "FUCK YOU, ASSHOLE."
A: "Sorry, I didn't realize that word was offensive."
12 different people: "Stop trying to backtrack, you piece of shit!"