What's the difference between a lateral calateral ligament sprain and a meniscus tear?

Ligament: The lateral collaterals are the ligaments that support the bones. The meniscus is the actual joint surface called cartilage.

Big difference: A sprain is a ligament which is usually stretched or partially torn. A grade 3 tear is a full tear in the ligament. Most sprains are treated with conservative measures such as ice, nsaids and therapy a meniscal tear is an injury to the "cushion" inside the knee which is between the femur and tibia. In general a meniscal tear will not heal and needs arthroscopic surgery to correct the problem.