The thighs are also commonly known as the 'quads' or quadriceps. This name is derived from the four muscles of the quadriceps. These four muscles are the rectus femoris (top of thigh to knee), vastus intermedius (outer aspect, lower portion), vastus lateralis (outer aspect), and the vastus medialis (inner aspect).
You will often see those performing thigh exercises using heavy weight amounts. The reason for this is because of all the muscles in the human body, the thighs are the most powerful muscle group.