The cruciate ligaments are the important central 'stays' of the knee, contributing significantly to the stability of the joint.
This illustration looks at the joint from the front when the knee is bent, displaying the cruciates deep within the joint. The cruciate ligament towards the front is the anterior cruciate ligament (ACL), and that towards the back is the posterior cruciate ligament (PCL).
This close-up on the left shows clearly the two ligaments crossing over. The word 'cruciate' means 'crossed'. The cruciate ligaments are also jokingly referred to as the 'crucial' ligaments, because they are so important in maintaining the stability of the knee (note that the bones are pulled apart a bit just to let you look inside).
The posterior cruciate ligament (PCL) attaches to the tibia at the back of the knee and the anterior cruciate ligament (ACL) attaches to the tibia at the front of the knee. Of the two, the anterior cruciate is the more important in this respect.
Note in the sketch on the left how the anterior cruciate ligament attaches, and the anatomy when the knee is bent and straightened. This is one of the reasons why ligament replacements do not do as good a job as the original ligament as this cannot be mimicked by the replacement.