Normal baby poop is soft and can vary: breastfed poop is often seedy and mustard-coloured, while formula-fed poop is a tan, pasty consistency. The key is that it's predictable for your baby.
True diarrhea is a sudden change to entirely watery, explosive stools that occur much more frequently than usual. If you see this, especially in a newborn, contact your pediatrician immediately to prevent dehydration.