The lats are a large flat muscle that has origin points at the mid and lower spine, the iliac crest (pelvis), and the ribs. Well-developed lats give the appearance of a wider back, therefore making the waist look smaller.
As far as functions, the lats have many, including shoulder extension (any time you do a pulling motion), shoulder adduction (the act of drawing your upper arm toward you), shoulder internal rotation (twisting your arm until your elbow is facing the ceiling) and lateral flexion of the spine (bending your torso from side to side).
Additionally, the lats also assist in hyperextension of the spine and anteriorly tilting the pelvis (the act of arching your back and poking your butt out).
In short, the lats are active during any pulling motion, and also during the arching and side-to-side bending of the spine.