A Mal-Occlusion literally means bad bite, or in other words any sort of problem with the way the teeth come together.
This could be a jaw discrepancy or a tooth problem. Teeth could be missing or there can even be extra teeth.
There can be many causes although the most common reason is poor "oral posture" or myo-functional habits. Genetics can be a factor, and there can also be sytemic conditions e.g. where bones and teeth can be affected.
Orthodontically Malocclusions are divided into 3 categories - based on how the molars relate to each other. Class I is normal and this type of malocclusion usually has the jaws in the correct positon but crowded and crooked teeth. Class II has the upper molars ahead of the lowers and may give you "buck teeth", and is usually a result of the lower jaw being too small or too far back in the face. Class III has the lower molars ahead of the uppers and can give an "undershot" jaw - this is usually due to a small top jaw, although about 20% of cases are due to a big lower jaw which can be inherited.
This classification is called the "Angle" system named after Edward Angle. The main problem with this system however, is that you can have a "perfect" Class I occlusion but set too far back in the face. This leads to a less attractive face and often to snoring and the potentially life-threatening Obstuctive Sleep Apnoea. Progressive Dentists and Orthodontists will be looking to try to prevent this.
Malocclusions can be "deep" leading to a short face with a square jaw, or "open" with a steeply angled jawline and a long face.
If intercepted early enough the severity of most malocclusions can be significantly reduced or even eliminated.