The main muscles worked are the quadriceps but lunges also work the hamstrings and glutes.
First, make sure that you have enough room in front of you to perform this exercise. Next place the bar across your back, just like performing a squat. Next, take a big step forward with your right foot, while keeping your torso upright and hold the muscles of your core tight. Next, squat down through your front leg until the knee of your left leg nearly hits the ground. Make sure that your knee does not go past your toes, this will prevent injury. Next, push through the heel of your right leg and bring it back to the starting position. Now, you can repeat this movement for a certain amount of reps, and then switch legs. Or you can switch legs each repetition and complete a certain amount of reps.
Make sure you start light and maintain your balance throughout this lift. Your form is very important to prevent injuries.
There are a many variations of this exercise, one would be walking lunges. This is where you pick a distance and keep moving forward through your lunges rather than bringing your legs back together.