Loose stool, or an increase in volume or frequency signals diarrhea. Causes of diarrhea range from parasites to viruses, from raiding the garbage can, to systemic illness. An occasional bout of diarrhea is not usually worrisome. However, if it doesn't clear up within a couple days, keeps recurring, or if your pet acts sick, it's time to seek veterinary help. If you look down and see white wigglers, think worms.
Tapeworms look like white rice or, if dried out, like sesame seeds.
Roundworms may look like spaghetti. Giardia, coccidia and other parasites may cause loose, cow pie-type stools containing mucus or blood. Parasites can be transmitted to people so use proper hygiene when cleaning up. To diagnose what creature is inhabiting the intestines of your pet, take a stool sample to your veterinarian. She will take a peek at it under a microscope and see what's there.