It works the bottom of your chest. Read Flat Bench Press for instructions. With arms slightly bent at the elbows, lying supine on the bench, in a circular motion, bring the weight down below a line going through your chest and parallel to the ground. Actually, bring the weight as low as you can without compromising your form. Don't try to cheat by going too low on your way down to build up momentum on your way up to pass through the sticking point.
Flies are good for both inner and outer pecs.
Close Grip Bench Press works the inner part of your chest. Do not take the bar too close however, leave at least two fists worth of space in between your hands. If you lower the bar closer to your neck, that will work the triceps. Then lift all the way up and repeat. Pull Over is a great all around exercise.