What ingredients are in canned cat food?

It depends on the quality. A good quality canned food will have a high meat content and with no cereals or grains (cheap filler that cat's cannot digest).

Some brands will include rice and vegetables. While cats, being strict carnivores, don't really need these, such ingredients are far better than grain.

Other things that can be found in cat food are:
Cereal by-products ("waste" cereals thrown away by the human food industry. This serves no purpose for cats. Found in lower-quality food.)
Meat and animal derivatives (this is the leftover bits of an animal such as feet, fur, feathers, lungs, intestines etc. Found in lower-quality food.)
Fats and Oils (such has omega 3. This is found in most qualities of pet food.)
Animal Fats
Vitamins and Minerals (added to the food that would otherwise be lost in the cooking process. This is found in ALL qualities of pet food.)