The Push, Pull, Legs split has been popular for decades. It's an excellent way to split up your workout that gives you a good amount of time for each muscle to recover before you train it again.
This routine is ideal for people who have 6-days to work out per week. If you have 5-days then you might choose to drop out on the 2nd legs day each week.
Day 1: Push
The first day is a push day, all exercises will be in a 'pushing' motion, training your chest, deltoids and triceps.
Bench Press: 5-6 reps x 3 sets
Dumbbell Shoulder Press: 5-6 reps x 3 sets
Incline Dumbbell Fly: 10-12 reps x 3 sets
Dumbbell Lateral Raises: 12-15 reps x 3 sets
Tricep Dips: 3 sets of maximum reps
V-bar Tricep Pushdown: 8-10 reps x 3 sets
Day 2: Pull
The second day is a pull day, all exercises are in a 'pulling' motion and will train your lats, traps and biceps.
Pullups: 4 sets of maximum reps
Barbell Row: 5-6 reps x 3 sets
V-bar Pulldown: 10-12 reps x 3 sets
Barbell Curls: 8-10 reps x 3 sets
Dumbbell Hammer Curls: 8-10 reps x 3 sets
Dumbbell Shrugs: 10-12 reps x 3 sets
Day 3: Legs
Day three is a leg day and you'll be training your quads, hamstrings, glutes and calves.
Barbell Squat: 5-6 reps x 3 sets
Leg Press: 12-15 reps x 3 sets
Leg Extensions: 12-15 reps x 3 sets
Leg Curls: 12-15 reps x 4 sets
Standing Calf Raise: 12-15 reps x 4 sets
This is a really great routine for intermediate and advanced lifters. Most beginners would be better off with a routine that has greater frequency and more compound movements such as Starting Strength.
As with most weightlifting routines, you should be progressively overloading. For any exercises where there is for example 8-10 reps, you should progressively increase each week in reps and once you reach the upper limit you should increase the weight by a small amount, which should drop you back to the lower limit.
There is no real need to make any changes to this routine, but if you wish to throw in some ab exercises or some additional exercises then you could throw that in on your leg day.