Diarrhea is a constant feeling like you have to poop and then trying to go and being unable to, all the while pooping out some blood because the body is weird. I had diarrhea on my eighth birthday, worst day of my life, well one of the worst days of my life. Looking through Wikipedia, what I am describing is more severe diarrhea, so I guess that the less severe kind is just a generalization for loose stool. Eating fiber makes poop softer and easier to process. I know everything about pooping. I guess stomach acid could cause something similar, but at least with me it is too much bile production (which also happens to be what makes poop brown in color, otherwise it would be a white-ish color).