Power showers are exposed mixer showers (with a hot and cold water feed) that also have a small pump inside the main box unit to boost the pressure which means it would require an electrical feed. They tend to only have a slider rail kit and handset (no overhead shower). You can get both Manual (no constant regulation of temperature) and Thermostatic (constant regulation of temperature) versions of these.
These type of showers are reducing in popularity in the bathroom market as many new boilers being fitted run high pressure systems. Also Digital showers can be a preferred alternative as although more costly, they can look a look smarter (no big box on the wall in the shower) and have the option for additional drench shower heads or bath fillers.