Also know as Rage Thread, these comics are made in the ultimate tool for cool graphics: MS Paint, and started on 4chan.org as a question/answer comic, where the first one was the question and the second... uh... the answer.
The comic was about a guy who was sitting in a toilet, and the weight of the turd splashed his butthole with his own urine and toilet water.
The answer came from another 4chan user, who suggested using toilet paper in sort of a "safety net" for the turd.
They're usually 4-framed comics, that always end with the same drawing: the FFFUUU graphic, showing some kind of frustration/pain/pissed-off feeling.
I've seen many variations of these comics, and sometimes there are more than 4-frames: 6, 8, 14!, and it also could happen the final face is a morph of the original face either smiling, race-change, gender-change, etc.
The drawings usually depicts frustrations that we all suffer, such as opening a beer can and get sprayed out. I think that's the most funny thing about them. They show something that could happen to any of us.
Well, I'd have to ask the guy who made the first one, but I'd say it's the FUCK word cut in half, hence the "-" at the end of the letters. Originally, the FFFUUU comics have 21 F's, 22 U's and an hyphen (-).