The word 'cruciate' means 'crossed'. The 'cruciate' ligaments are so-called because they cross over one another as they link the two bones together. The illustration on the left shows a bent right knee, with its corresponding bones. You can see that the cruciate ligaments are situated right in the middle of the knee.
If you bend the knee and look in from the front, and flick the patella (kneecap) out of the way (image on the right), you can see the cruciate ligaments from the front through the notch between the two rounded ends (or condyles) of the femur. The ligament you can see from the front of the tibia is called the anterior cruciate ligament, and the posterior cruciate ligament lies behind it.
A true ligament is a supportive band that connects one bone to another bone. It is made up of strong fibres (collagen) and supports the bones of a joint from dislocation. A ligament has a certain amount of elastic stretch, but can weaken if too much tension is put onto it.
This illustration demonstrates how the fibres of the cruciate ligament are attached (imagine this right knee is cut vertically through the notch, and you are looking at the ACL from the inner aspect). Not only do the ends of the ligament splay out to gain a wide contact surface, but the ligament itself has a twist in it, allowing a complex mechanical action which allows some ligament fibres to take the strain in flexion (bent) and others to take the strain in extension (straight).
Any graft needs to be placed in a similar line where it acts most efficiently, not getting too slack or too tight as the knee bends and straightens - surgeons call this the 'isometric point' - and complex jigs are used to place the graft in the optimal position.