TrainingPeaks' Training Stress Score (TSS) system allows any endurance athlete the ability to quantify their workouts based on their relative intensity, duration and frequency of workouts.
Normalized power provides a better measure of the true physiological demands of a given training session - in essence, it is an estimate of the power that you could have maintained for the same physiological "cost" if your power output had been perfectly constant (e.g., as on a stationary cycle ergometer), rather than variable.
Intensity Factor provides a valid and convenient way of comparing the relative intensity of a training session or race either within or between riders, taking into account changes or differences in threshold power.
Watts per kilogram is the standard measurement of a rider's "power to weight ratio" and represents a rider's average power output over a period of time, divided by the rider's body weight. w/kg equalizes power output between riders of various sizes and, all else being equal, increasing power (watts) or decreasing weight (kg) will make a rider faster.