Biggest danger of diarrhea is dehydration. Give your dog electrolyte enhanced water like pediacare or sports water. Dehydration is indicated if you pinch your dog's skin (try the neck/ruff area or if it's a hairy breed try this on a leg) and the pinched area doesn't immediately spring back into place, i.e. Leaves a little ridge where you pinched. I'd take my dog into the vet after two days because if the diarrhea is related to something s/he ate it should have flushed itself already. Could be something more serious.

 Diarrhea in dogs is due to number of reasons like if dogs have eaten something disagreeable, eaten too much, too fast or exercised too soon after eating, are overly excited or nervous. The following are the conditions when you should see your vet;
* bloody diarrhea
* diarrhea accompanied by vomiting
* multiple bouts of vomiting occur over a short period of time
* ingestion of a poison is suspected
* fever and other signs of toxicity are present
* diarrhea persists more than a day or two
* dog also appears listless
* there is weight loss
* dog is showing other signs of illness such as labored breathing or pain

 Give your dog a bland diet: Boil hamburg and white rice. Mix the two ingredients and give it to your dog, in the same serving size as your would its dry food. After a day or two if he still has the issue take him to a vet to observe. He could need an antibiotic. Take caution near the feces because it may be transferable to humans if it is a particular virus.