Can Somebody Explain to me What is a Power Resistor?

A power resistor is simply a resistor which is designed to be used in high voltage/current situations (usually mains voltage or above). Since the power dissipated by a resistor is proportional to the square of the voltage across it (see Ohm's Law), special precautions need to be taken so that the resistor can dissipate the power without burning out. Resistors are built with a specified power rating. If you double the voltage across a resistor, you will have to dissipate four times the power as heat.

All resistors have a power rating. The more common through hole variety are rated at 1/4W for the E12, E24 range, etc. If you go surface mount, they are more likely to be 125mW for a 1206 case style & smaller - with smaller power ratings for smaller sizes. The term Power Resistor is somewhat subjective. I would probably class anything rated over half a watt & of current manufacture to be a power resistor. Generally, power resistors have compromised other ratings, such as tolerance & temperature co-efficient because they are designed to dissipate larger amounts of power in the form of heat. Good modern design practice aims to eliminate such wasteful & inefficient components. The most noticable way they are different from the typical component in terms of physical appearance is that they are generally much larger.