Endoskeletons, as Webster noted, are skeletons found inside a body, as in the case of most land animals other than insects. There are several advantages and several disadvantages of having this type of skeleton.

The main advantage of these types of skeleton are that they are flexible. Animals with exoskeletons are generally less able to move about than animals witn endoskeletons. Also, animals with endoskeletons are more able to grow, and can grow larger. Exoskeletons are very constrictive; animals with an exoskeleton must shed it periodically as they grow. As their new one forms, they are very vurnerable. Animals with endoskeletons are immune to this problem. Also, animals with endoskeletons can grow to a much larger size, which is why you don't see 15 foot tall insects running around.

There aren't too many disadvantages to having an endoskeleton, which is why most large animals around have them. However, one large one is the vunerability animals face without the exoskeleton. Animals with 'smooshy' skin are vurnerable to predators, dessication, heat, cold, etc, to a much greater extent than animals with exoskeletons. However, in such a diverse world, animals with both type of body form are able to thrive.