The meniscus is a C-shaped cartilage located between the knee joint and it can tear with sudden unaccustomed twisting movement. It provides stability and cushioning to the joint. We have a medial (inside of the knee) and lateral (outside of the knee) meniscus. Each meniscus has an anterior and posterior horn. The most common meniscal tear occurs in the posterior horn on the medial meniscus.
Meniscal tears often occur due to unaccustomed twisting and rotational movements to the knee that impact on the cartilage e.g. during a tackle in football. This is known as an acute meniscal tear. They can also occur for no reason, especially as we get older and these are often called degenerative meniscal tears.
There are many different types of meniscal tears. An MRI scan is the only way of knowing what type of tear you have sustained.