The back is made up of many different muscles, but the routines that follow target the latissimus dorsi, rhomboids and the trapezius muscles.
If you build up these muscles and improve their shape and definition, you create a well proportioned back with a broad muscular shape.
Generally, people aim for one or two effects when building a muscular back: a wide back with an open V-shape or a thick-set back with greater girth.
For a wide back, concentrate on pull ups and lat pull downs that emphasise the latissimus dorsi muscles (the lats). For thickness, you need to concentrate on the lats and the rhomboids: reverse flys, seated rows and bent over rows will emphasise these muscles.