Under extension is a problem because your arms or legs never look straight. Whenever your teacher says "straighten your knees!," your reply is "I am!" This problem could be in the joint (structural), or it could be muscular. For example, maybe the bicep is always slightly activated and the tricep is too weak to fully counteract the bicep, resulting in a perpetually "bent" elbow.
Hyperextension (or "Over extension") is a lot more common and "easier" to fix. This is when your arms or legs seem to bend backwards, past the point of being "straight."
Bowed legs affect a good portion of the population, especially Asians. This is a condition where the legs are curved outward so that the knees do not touch on the inside when the person stands with their feet together.
Also not something curable, but mild bow-leggedness most likely does not put a person at risk for injury. The only thing a bow-legged individual needs to watch for is tracking their knees over their toes in parallel first position - it is hard for you to know where your knees are and the tendency is for them to drift apart.
All of the conditions I have listed above are mostly aesthetic - there is no way or reason to correct for any of these joint abnormalities. Some of the best dancers there are have these abnormalities and are very aware of them: in fact, it is the abnormality that makes them more aware and causes them to pay special attention to correct posture and stance at all times. While none of these necessarily create an "ideal" look, there is nothing wrong with them. Some choreographers may even exploit these abilities (some really love hyperextended elbows. It's interesting). Love the body you've been given.