These articles explain it very well:
Strength Training or Resistance Training is a form of exercise that trains each muscle targeted from Origin to insertion of the muscle fiber.
You can choose to add resistance to the muscle by using dumbells or machines or body weight.
Ideally strength training should be done three days a week…every other day..or at least two days a week.
Choose each muscle group working the agonist and antagonist. Execute 10 to 12 repetitions to begin…using the method of lifting called T U T….Time Under Tension.
This means…lifting the weight slow and controlled through the range of motion.
Strength training can help you achieve a higher metabolic rate…ie how many calories your body burns at rest.
The more muscle fiber you build…the higher your metabolic rate.
Strength training can protect the joint from injury by surrounding the joint with protective fiber.
Strength training can aid your Cardiovascular workouts by making you a Stronger more powerful athlete.
There is a difference in Strength and Power…strength is the ability to move an object or keep it from moving. Power is the ability to Move that object as Fast as possible.
A more advanced Periodization program can be built from a Progessive and well overloaded Strength program.
Okay, let’s break this down to first acknowledge what “strength” is. It is the amount of resistance a muscle or group of muscles can maneuver concentrically or eccentrically. So, training this is as simple as Heika said “… working against resistance of any kind” enhancing the muscle or group of muscles ability to maneuver the given resistance.