Ligaments are the fibrous, slightly stretchy connective tissues that hold one bone to another in the body, forming a joint. Ligaments control the range of motion of a joint, preventing your elbow from bending backwards, for example, and stabilizing the joint so that the bones move in the proper alignment.
Ligaments are composed of strands of collagen fibers. While ligaments are slightly stretchy, that they are arranged in crossing patterns prevents the joint itself to become loose.