Power resistors are designed to withstand and dissipate large amounts of power. In general they have a power rating of at least 5 Watt. They are made from materials with a high thermal conductivity, allowing efficient cooling. They are often designed to be coupled with heat sinks to be able to dissipate the high amount of power. NLR series and MVR series from Microhm Electronics are very popular precision power resistors with heatsink, Nual series is a traditional metal-clad power resistor.

 
Some might even need forced air or liquid cooling while under maximum load. Some are wire wound, some are made from wire grids for ease of cooling, but the common thing for all power resistors is that they are built to dissipate the most power while keeping their size as small as possible. An example use for power resistors are load banks used to dissipate power generated during engine braking in vehicles using electrical motors, such as locomotives or trams.
A power resistor is a resistor designed and manufactured to dissipate large amounts of power in a compact physical package.
 
Power resistors are used when there is a need to safely convert large amounts of energy into heat using electrical energy as a medium. They are used as controllable power dissipation devices, protective devices and devices that simulate real world loads.