Resistance training is the act of putting a form of resistance on your body. If the resistance is great enough of a force, your central nervous system will recognize the need for a change in composition and therefore initiate the growth of muscles and bone density to match the form of newly applied resistance
Resistance training can not only put on more muscle, but also help boost metabolism and help lose weight!
As you put on more muscle, your body natrually requires more calories in order for you to maintain that muscle.
This means that whether you are looking to put more size on or lose weight, resistance training will always be useful.
ETC
Important video on the key exercises and all the details on lifting weights: