The meniscus is a specialized structure that is thicker where it attaches to the lining of the joint and thinner toward the middle of the joint. If you made a cut across it, the meniscus would be triangular or pie shaped.
The meniscus can tear in a number of ways. Tears can result from a sudden twisting-type injury or can occur gradually with age. The tear can be through either the outer thick part or inner thin part. Some tears involve only a small portion of the meniscus, while in others nearly the entire meniscus can be involved.
Meniscus tears can cause symptoms from portions of the torn fragments getting stuck or pulled in between the bones as the knee moves. This can cause snapping or popping, locking, pain, and swelling. Not all meniscus tears cause problems, but when they do the arthroscope can be used to trim out the piece which is torn. Only the portion which is torn is removed.