Running has been touted as the biggest "bang for your buck" when it comes to efficient calorie burn. This essentially means that running is the best form of cardio to choose if you want to burn the most calories in the shortest amount of time. This is mainly because running uses muscles all over the body. Obviously, the main muscles used to propel the body forward during the running motion are the legs. The quads are engaged the most, as they take the most of your body weight upon impact on the ground during your foot strike, and then are employed the most to propel you back off the ground moving forward. However, the hamstrings (back of the thighs), calves, hip flexors, glutes, and the IT band (that runs down the sides of the legs from the hips to the knees) are used quite a bit as well.
But to run well and efficiently, the body must have a strong core too. Running requires an upright posture, so abs and back muscles are steadily engaged. Not only that, but running requires swinging your arms forward and back with a 90 degree bend at the elbow, so the shoulders and biceps get some work in too. Essentially, running is a full body workout with an emphasis on training leg muscles.
But what does this mean in the grand scheme of things? Well, along with a mostly healthy diet and getting enough sleep and rest, running can change your physique to be toned, lean, and muscular. The body burns more calories to maintain muscle than it does to maintain fat, so the more muscle you build on your body, the more calories you will burn and the more efficiently you'll lose weight. In addition, you might find an increase in your appetite and hunger levels, as more muscles require more food and signal to your stomach to be fed more frequently. You might find that your clothes fit looser, or just differently, as certain parts of your legs are built up and others slim down.