This is pretty much the easiest form of exercise there is: you can walk anytime you want, anywhere you want, and no equipment is required. It doesn't have the learning curve that some forms of working out have, and you can work it into your daily life.
Again, you can target your glutes by walking uphill, and if you walk more quickly and more energetically, you'll use up more calories. This is great if you want to burn fat.
An hour of brisk walking burns about 300 calories for a 140-pound person.