Diversity is a measure of how many kinds there are, but not a measure of how different they are from each other. Disparity is a measure of the amount of morphological differences between 2 or more species. This question arose when scientists debate about the Burgess Shale fauna. Stephen Jay Gould points out that the Burgess Shale invertebrate fauna, dated a few million years after the Cambrian explosion, is much more disparate than the living fauna. His opponents disagree.
A good example of diversity is the number of species of frogs and toads (Anura). Because there are thousands of species of frogs and toads, they are said to be a diverse group. In contrast, apes are not a diverse group because there are only 5 species of great apes (orangutan, mountain gorilla, bonobo, lowland gorilla and common chimp). Frogs do differ from one another, but there is not a great deal of difference among all of the different species of frogs and toads. Each of them have 4 limbs and no tail, so even non-scientists can recognize all of them as either frogs or toads. In terms of disparity, a caterpillar and a butterfly are more disparate from each other than practically any 2 frog species, even though the caterpillar and the butterfly may be the same species. The caterpillar, for example, has no wings, unlike the butterfly. Without knowing that the caterpillar will transform into a butterfly, many people would have thought that they represent not only different species, but perhaps different genus or even phylum of animals. Tadpoles, too, are more disparate from a frog and any 2 frogs are from each other.
Frogs and toads, therefore, form a diverse but not very disparate group. The great apes is a group that is neither diverse nor disparate. In contrast, molluscs as a group is both diverse as disparate. There are over 90,000 species of molluscs, making it a very diverse group. Within the Mollusca, we find animals with shells (clams and snails) and without shells (e.g. octopus, squids and cuttlefish), with a shell that opens into 2 equal halves with a hinge in the center (bivalves), such as clams. Then we can also find shells that are coiled, with but one opening (the gastropods or snails). Even within the gastropods, we find both animals that have shells (snails) or those that don't (slugs). These differences make the molluscs a disparate group as well as a diverse one.
Finally, there are also groups that are disparate but not very diverse. The monotremes is a good example. The duck-billed platypus and the echidna are the only living monotremes, and the echnida is more disparate from the duck billed platypus than any 2 species of frogs and toads are from one another.