From FrugalReef

Invertebrates are some of the most interesting and varied of the reef inhabitants. "Invertebrate" means "without vertebra". Vertebra is name for the bones that make up the back bone of animals like fish, mammals, and humans.

For the most part, anything not a fish and not a plant in your aquarium is probably an invertebrate. This includes clams, corals, shrimp, crabs, etc, etc.