Power resistors' wattage rating is specified at a temperature of 25C. As the temperature of a power resistor climbs above 25C, the power that the resistor can handle safety begins to drop. Since 25C is a typical room temperature, and any power dissipated by a power resistor generates heat, running a power resistor at its rated power level is often very difficult. To adjust for the expected operating conditions, manufacturers provide a derating chart which shows how much power the resistor can handle as the temperature of the resistor goes up.  It is best to use the power derating curve as a guideline and stay well within the suggested operating area. Each type of resistor will have a different derating curve and different maximum operating tolerances.

Most electronics applications use low power resistors, typically 1/8th watt or less. However, applications such as power supplies, dynamic brakes, power conversion, amplifiers, and heaters often demand high power resistors. Generally, high power resistors are resistors that are rated for 1 watt or greater loads and are available in the kilowatt range.
 
The increased size helps to dissipate heat and often is used to provide mounting options for heatsinks. High power resistors are also often available in flame retardant packages to reduce the risk of a hazardous failure condition.
 
Several external factors can impact the power derating curve of a resistor. Adding forced air cooling, a heatsink, or better component mount to help dissipate the heat generated by the resistor will let a resistor handle more power and maintain a lower temperature. However, other factors work against cooling, such as the enclosure keeping heat generated in the ambient environment, nearby heat generating components and environmental factors such as humidity and altitude.