Meniscus injuries of the knee are often referred to as torn cartilage.  The meniscus is a cresent-shaped "shock absorber" which lies between the femur (thigh bone) and the tibia (shin bone).  There is one on the inner (medial) side of the knee and one on the outer (lateral) side of the knee.  The medial meniscus is more commonly injured than the lateral.

 

If a torn fragment of the meniscus is large, it will cause the knee to catch, lock or give way.  This occurs because the torn fragment gets lodged between the bones (acting like a door jamb), preventing the "door" from fully straightening or fully bending.