Lunges if done properly are really hard work, not only do they exhaust the fast twitch muscle fibers they also put the cardiovascular system to work as the sets are can be quiet long.
They are a leg exercise and more specifically your quadriceps but they also work the glutes (your bum) hard as well. Another couple of good points to note about lunges are that they challenge your balance and therefore your core muscles (which makes them a great lower limb rehab and sport specific exercise) and they isolate each leg separately (great if one side is lagging in terms of strength).
In the example below I’m demonstrating alternating lunges with a barbell on my back. You should try lunges with dumbbells in your hands first as with the barbell version the weight is further from the ground which alters your centre of gravity more than dumbbells and therefore makes the exercise (given the weight is the same) harder.
Performance: Set up a barbell with the desired weight on a squat rack or get the equivalent weighted set of dumbbells. Lift the bar onto your back or pick up the dumbbells in each hand. Lunge forward with one leg making sure your trailing knee drops as close to the ground as possible. Keep your head up and your back straight throughout. Then drive off the ground with your front foot returning to your start position. Repeat with other leg. Try to remain steady and take clean smooth steps when performing this exercise, avoid stumbling all over the place.