Weight training is beneficial for developing muscle mass and definition throughout your body's structure. As you can imagine there are hundreds of muscles and most are bilateral (found on both sides of the body). The main muscle groups that benefit from weight training are, biceps, triceps, forearms, lats, traps, delts, pecs, quads, gluts, calves, and hamstrings.
The delts and upper section of the traps are the muscles making up the shoulder area. The lats and lower traps make up the back muscle group. The pecs make up the chest, the biceps, triceps and forearms make up the muscle group supporting the arm. The calves, hamstrings, quads, and gluts are the leg muscles.
When you're weight training, you're making microscopic rips in the muscle tissue. Once you stop working out the muscle tissue begins to heal, and in the process the muscle increases in size and strength. With a proper, protein-rich diet this will develop mass and definition over time.
Free weights are mostly barbells or dumbbells. There are some machines, like the "Smith" machine, which consists of a barbell that is fixed within steel rails, allowing only vertical movement. Other machines exist as well that isolate the muscle groups for easier development. To develop core strength and to get the most out of each workout, it is better to use free form barbells and dumbbells.
Before trying to use any of free weights it is beneficial to have someone show you proper technique, stance, and posture, starting light and working your way to more and more weight. It is crucial to have proper supervision in the beginning. This is called, "spotting,” and it is an unwritten rule of the gym that if you ask the person nearest you for a spot they are inclined to help you.
In the next article I will go into some weight training exercises, "pyramiding", and a brief description of how to execute them and what parts of the body they benefit.