The Pecs can be broken up into 4 areas, but the biggest division is the upper and lower portion. If you look at a body builder, you will see that their chest muscles have a clear delineation between the upper and lower halves. This usually occurs right in the middle area around the nipple and moves all the way across the chest. This means that if you want to build a solid chest, you will need to find a way to hit both the upper and lower portions. Luckily, there are several ways that you can do this.
Using an incline bench with barbells or dumbbells will help you worth the upper portion of your chest specifically and is one of the easiest ways to do so. You can put the incline at about 60 degrees for best results. The decline bench press is a great way to hit the lower Pecs and works the same way as the incline. You can generally lift more weight with the decline press than the incline, so make sure that you put enough weight on the bar to really feel the strain.
Although there aren’t specific outer and inner portions of the Pecs per se, they do exist in theory. You can generally feel the difference when you are doing a chest flye on the machine or with dumbbells. You will feel it the next day on both the inside and outside of the chest. To make sure that your chest is well rounded, make sure that you try to do either cable or machine flyes once every week to fully develop the inner and outer portions of your Pecs.
The chest muscles, like most muscles in the human body, look like one single muscle, but they are actually broken up into a series of muscle fibers that work in different ways. If you look at a really tone person or a bodybuilder, you will be able to see the difference in each part of the chest. Bruce Lee is actually a great example of this; in fact you can almost see every one of his muscle fibers. The best way to understand any muscle is to see it outlined and then to feel the different parts at work in the gym. I hope this helps to give you a basic understanding of how the chest muscles work and interact with each other.