Cellulose is a type of carbohydrate that can be digested by ruminants and not by humans. Ruminants have a large sac-like structure between small and large intestine where the food containing cellulose is digested by the action of certain bacteria. On the other hand, humans cannot digest cellulose, as the cellulose digesting enzymes are absent in them.
That large sac is called Caecum. It is well developed in ruminants but not in humans. It contains bacteria which produce an enzyme called cellulase which digests cellulose.