Nic,
It's watery poo, and it has this smell, different to normal,almost acidic and sometimes there is vomiting (more than reflux), low grade temp. Sometimes they refuse their feeds. As Simone said, so long as they are not dehydrated, it's ok, it a virus and will pass, it's just gross and hard work when they are sad all the time and you feel as though there is nothing you can do for them.
My paed said six good wet nappies, fontanelle that feels normal, so not depressed, wet mouth and tongue are all good signs.
Really dehydrated babies become lethargic and their hands and feet get cold, their eyes have a sunken appearance, often appear grey. HTH