The muscle is a large group found on your back and referred to by the Greeks as broad muscle of the back. It is a fairly flat, dorso-lateral muscle attached to the trunk that is partly covered by the trapezius muscles. It has many functions including those with your spine and shoulder as well. The main functions of the lats are to pull the arm down towards the pelvis, and in hanging positions to bring your body up towards your arms.
The lats as they are commonly referred to as are the muscles that most people would assume make up your complete back. The areas that hang on either side of your body and are so pronounced in athletes. There are many exercises that target this group and in many different goals such as to add extra width or improve strength.