Tuataras have a bodyplan that superficially resembles that of a lizard. Most rhyncocephalians had this type of bodyplan, thus giving one an idea of what the ancestral lepidosaurian bodyplan was. Tuataras have four limbs, with five digits on each one, a tail with the ability to be broken off and regrown and a pineal or "third eye" on top of their heads. All typical of many lizard species.
But here's where things change.
First off tuataras lack external ears. They have hooklike extensions on some of their ribs (which can also be found in birds) and males lack a penis (another avian convergence).
The tuatara's pineal eye or "third eye" is more developed than those of lacertilians. The "eye" is comprised of a lens, retina and nerve connection to the brain. In lizards this pineal eye is used for temperature regulation, giving the animals a fairly good idea of the surrounding temperature and allowing them to know when they are too hot or too cold. Since tuataras are nocturnal forest dwellers it is harder to say what their pineal eye is used for. If it is used for hte same reason as those in lacertilians, then why is it more developed?
The teeth on the upper jaw are fused to its summit and cannot be replaced. Furthermore, tuataras have an extra set of teeth on their palates that no lizard has.
Males and females are sexually dimorphic with males being much larger and heavier. Males also have prominent nuchal and dorsal crests along with narrower abdomens than females.
Tuataras are also capable of caudal autotomy, just like many lizards.
The palatal teeth in tuataras run parallel to the marginal teeth and fit in such a way that the teeth of the lower jaw fit snugly inbetween the twin rows on top. This snug fit allows for a self sharpening action which always keeps the teeth from dulling out. When eating, the tuatara will actually slide its jaw back and forth, grinding up its meals. This back and forth chewing is different from the rotational side to side like chewing found in mammals, showing us that this trait was independantly evolved in at least one family of rhynchocephalians.