Most people associate Heavy Weight with Low Repetitions for mass and Light Weight High Repetitions for definition. In the medical world we associate Heavy Weight and Low Repetitions with injury.
Let's break this down. Our body adapts to the stress we place it under. The heavier the weight the less times you can actually lift the weight compared to a lighter weight. Continuously lifting heavy weight will cause your body to adapt, therefore strength gains will follow. In comparison, lifting a lighter weight for 15 - 20 repetitions will cause the body to adapt, allowing you to lift more weight for 15 - 20 repetitions.
In the gym our body will generate energy to move weight through one of two systems, depending on the system targeted. The Creatine System is designed for powerful, explosive movements over a small amount of time (moving heavy weight). The Glycolytic (glucose / sugar) System is designed to last a little longer (moving lighter weight).
Using the knowledge above, if your goal is to acquire as much mass as possible, with little concern about definition, then heavy weight with low repetitions should be your choice. Remember to use strict form (see additional articles on proper form found in this website) to prevent injury. Rest times should be 2 - 3 minutes to allow the Creatine System to replenish itself before moving on to the next set.
For those wanting a more defined and cut look, use a lighter weight with higher repetitions. Utilizing the Glycolytic System allows your body to burn more sugar for energy, instead of being stored as fat. Less rest time is required. The higher your heart rate the better. Once stored glucose reserves are depleted your body will turn to a 3rd system, the Aerobic System, to utilize fat for energy.
Good luck and lift safe