I'm an absolute "ectomorph"

• ECTOMORPHS are generally small-framed and thin. Their shoulders and hips are generally narrow. They have trouble gaining weight and building muscle. Ectomorphs should do enough cardio exercise to work their heart and lungs but not so much if they are concerned about building muscle and maintaining weight. Weight workouts should be on the heavier side if the goal is to build muscle. Weight training the upper body is helpful to keep proportions more even. Generally, lighter weights and more reps do the trick. Ectomorphs are generally tall and thin and have long arms and legs. These people have difficulty gaining weight and muscle no matter how much they eat or how hard they weight train. They have the body type you tend to see in ballet dancers, runway models, long-distance runners, and some basketball players. A very small proportion of the population has this type of body.
• MESOMORPHS tend to be of normal weight, neither fat nor skinny. They tend to pack on muscle easily and can lose weight easily up to a certain point. It is hard for them to become "skinny." Generally, their shoulders are wider than their hips. Mesomorphs should do longer-duration, higher-intensity cardio work if the goal is to lose weight. Building muscle is easy, but if the goal is not to gain size then moderately heavier weights and higher reps will be the way to go during weight-training sessions. Mesomorphs are generally muscular, shorter, and have stocky arms and legs. These people are strong and tend to gain muscle mass when they do strength training. They may find it difficult to lose weight, but they excel in power sports like soccer, softball, vaulting in gymnastics, sprinting events in track and field.
• ENDOMORPHS tend to be pear-shaped, meaning their hips are usually wider than their shoulders. They gain weight easily and tend to be curvier than the other body types. Endomorphs are generally shaped like apples or pears and carry more body fat. Their bodies resist losing weight and body fat no matter how restrictive they are with their eating. In fact, the more they "diet," the more their metabolisms slow down to resist weight loss. These people are better able to handle long periods of starvation and famine (which was a benefit to our ancestors). Sports they excel at are distance swimming, field events, and weight lifting.
  

Most of us aren't purely one body type or another; we are a combination of at least two. That means you're going to have to adjust your workout to strike the right balance of exercise and diet. I find it helps to keep a food and workout diary so you can more easily analyze what works and what doesn't.