The normal knee functions like a hinge. The knee is not a true hinge because, in addition to movements of flexion (bending) and extension (straightening), the knee is capable of small degrees of rotation. Failure to recognise this detail led to ctastrophic failures of some of the earlier designs of total knee replacements.
Full extension of the knee as in standing erect is referred to as 0 degrees. From this position, the knee will bend approximately 135 degrees. Further movement is stopped by the thigh and calf coming together. Obese people therefore have a smaller arc of motion.
Some people have knees that "bend backwards" by about 10 degrees. This is typically seen in double jointed people and there is not much clinical significance to this in isolation.
The quadriceps muscle in front of the knee extends the knee and the hamstring muscles behind the knee flex the knee.
The knee joint absorbs large forces generated by normal muscle action in day to day activity. The femur and tibia are not a perfect match and the total contact area between these two bones is increased about three fold by the intervening menisci. It is easy to appreciate why damage to a meniscus will increase future risk of arthritis.