An electric torque wrench is a gearbox that is connected to an electric motor. This gearbox is designed in such a way that it absorbs the torque, allowing the user to operate the wrench with minimal effort. These are typically used when there is no need for torque strength accuracy being used on the nut or bolt, or when a very stubborn nut needs to be removed. 

The gearboxes can be of ratios of up to 4000:1. and this is adjusted by controlling the voltage. Due to the fact it is powered by electricity, you can find waterproof models for when water is a concern on the jobsite. It is often confused with an impact wrench due to its similar design, but it is in fact a totally different tool, and is even more different from a hydraulic torque wrench. It all comes down to the job and getting the right tool for it.