One of my favorite exercises for building back muscle is wide grip pull-ups. I often start off my back workouts by doing as many pull-ups as I can until I reach 50 reps - this was something I got from Arnold's Encyclopedia of Bodybuilding. It's a fantastic way to build strength, endurance, and width into your lats. There are plenty of back muscle exercises, below are a few that I've liked using as part of my back muscle workouts.
Deadlifts - this is not just a back exercise, but an entire body compound exercise. Almost every single muscle group is used during a dead lift. You don't see many people do these in the gym - perhaps the name scares them away. Nonetheless, deadlift is one of the best overall strength builders that you can perform.
Pull-ups - This exercise works to add width to the upper back and create a full sweep in the lats. Start off with just using your body weight, and if you find it difficult at the start, you can perform assisted pull-ups either with a spotter, or using a pull-up machine. As you progress, you can eventually perform weighted pull-ups by tying weights to your body while performing pull-ups. Variations include behind the neck pull-ups and close grip chin-ups.
Bent over barbell rows - barbell rows works to thick the upper back muscle and also helps to add some width to the lats. Other variations and similar exercises includes bent over dumbbell rows, one arm dumbbell rows, and T-bar rows.
For lower back muscle development, deadlifts, hyper extensions, good mornings are all good exercises that targets the lower back muscles. Your back muscle workouts do not need to long. All you really need is three or four good exercises per workout, and this should take no longer than 45 minutes. Each exercise should involve 3 sets of 4 to 6 reps - you need to use a heavy enough weight that will stimulate muscle growth and strength development.
Here's a sample back workout routine:
Pull-ups - 50 reps (perform as many reps as you can each set, until you reach 50)
Deadlifts - 3 sets x 4 to 6 reps
Bent over Barbell Rows - 3 sets x 6 reps
Hyper Extensions - 3 sets x 8 to 10 reps