A forward lunge targets the abdominal muscles (transverse abdominus and obliques), the glutes (gluteus maximus, gluteus medius/minimus), the anterior thigh muscles (quadriceps) and the posterior thigh muscles (hamstrings). According to Healthy Living, it also works the calf muscles.
According to MuscleMagFitness.com, performing a forward lunge involves stepping forward with one leg, then bending until the thigh is parallel to the floor and the back knee almost touches the floor. The movements are then repeated using the other leg. Details of positioning during this exercise dictate what muscles are uses. Taking a longer step works the hamstrings more, while taking a shorter step uses the quadriceps more.