IBD refers to inflammation and damage to the digestive system. In contrast, IBS is a functional disorder that affects the motility of the gastrointestinal tract, causing uncomfortable symptoms like abdominal pain, bloating, diarrhea and constipation.
Unlike IBD, IBS does not necessarily involve inflammation or damage to the digestive system. It's also more likely to affect the large intestine than the small intestine.
That said, it is possible to have both IBD and IBS together, though IBS is more common in humans than dogs.