While weight training will increase muscle size and strength, most of you still want to get into the best shape possible. Many weight trainers will try to gain as much weight as possible to add more muscle. The only problem with this is that you'll likely gain fat as well as muscle. While you may have a nice amount of muscle mass you may have a layer of body fat covering up all of your hard work. To get the defined, ripped look most men strive for you'll need to lose the extra body fat.
How much body fat you hold does depend a lot on your body type. If you're an ectomorph (naturally skinny, long limbs) you'll have an easier time losing body fat, or not gaining it at all, but a harder time building muscle. If you're a mesomorph (naturally athletic and muscular) you should be able to lose fat and build muscle easier than people with other body types. If you're endomorphic (tend to hold extra body fat) you should be able to build muscle mass but will likely have a tougher time getting rid of body fat. Because of this, endomorphs need to watch their diets and do more cardio workouts to burn body fat. However, no matter what body type you have you'll want to lose extra fat if you have it.
Building muscle and losing fat at the same time is virtually impossible to do. This is why many bodybuilders gain a lot of weight, both in muscle and fat, when they aren't training for a contest. The added fat will help you get stronger and consequently build more muscle size. When you first start weight training you may actually gain muscle and lose body fat at the same time, but it won't always work out that way.
When you decide to get more muscle definition you'll need to lose the body fat with the right diet and more cardiovascular exercise. If you consume less calories than you burn you'll start losing body fat. It's also a good idea to eat smaller meals more often whether you're trying to build muscle or lose fat. This will boost your metabolic rate, which is very important to fat loss. Consuming less calories in the form of carbohydrates will also go a long way in helping you get rid of the fat. Keep most of your carb intake earlier in the day and avoid consuming too many of them later. Of course your diet needs to be nutritious and high in protein.
Cardiovascular exercise will also help burn more calories. Running, jogging, biking, boxing workouts, elliptical machine workouts, and playing a physical sport are all great forms of cardio training. Try to work your way up to cardio sessions that last 30 to 45 minutes 4 or 5 days a week to burn fat. To make your cardio more effective you should do it first thing in the morning on an empty stomach or possibly after you weight train. Doing cardio during either of these times is good for fat burning because you'll be in a glycogen depleted state so your body won't have to burn as many stored carbohydrates before it starts burning fat.
Weight training itself will give you a higher resting metabolism because gaining muscle mass will force your body to use more calories to sustain it. This is why you may not gain as much fat as you did before you started weight training even if you eat the same amount of calories or a bit more.