Electric Shower.
Heats water electrically as you use it. Connected only to your cold water main but does require a heavy duty electrical connection.
You can shower whether or not there is hot water available from your boiler.
Mixer Shower
This blends hot and cold water to provide your chosen shower temperature. Pressure requirements vary but this type of shower works best on a high pressure system. If you have a gravity system a pump might be required. The picture shows a built in mixer valve which is easily installed in a stud wall.
Mixer showers can also be surface mounted like this bar style model. These are usually used on a solid wall where the built in type cannot be fitted.
Ideally look for a thermostatic
mixer valve where the water temperature is maintained thermostatically.
If another tap is used , or a toilet flushed the water will not suddenly
run freezing or scalding hot.
Pumped Shower, (Power shower).
This requires a gravity hot and cold supply and uses a pump to boost your water pressure, giving a really powerful shower. It cannot be used with a combination boiler or an unvented system.
The one pictured here has an integral pump and is very easy to install.
A shower pump can be used with any mixer shower to boost your water pressure. Again it cannot be used with a combination boiler or an unvented system.