A common definition of invertebrate is "an animal without a backbone, or spine." But the more correct definition is an animal without a notochord. A notochord is a rigid rod made of cartilage that is present in all chordates.
What Do Invertebrates Look Like?
Other than the lack of a notochord, it is difficult to state characteristics common to all invertebrates, since there is such a variety of species. Invertebrates make up a huge majority of the world's species - most invertebrate articles list invertebrates as making up 95-99% of the world's species. With so many species, there is great diversity in the form of invertebrates. Some have many legs, some have no legs. Some have shells to protect them, some don't.
Where Are Invertebrates Found?
Invertebrates are found all over the world - on land, in freshwater, in salt water and many different habitats. This is true for invertebrates in the ocean - they may be found easily in tide pools, but are also found in the deep sea, and may inhabit both warm and cold water.