This type of plumbing system comprises a hot water cylinder (often situated in an airing cupboard) being fed with cold water from a header tank (usually located in the loft of a house or building).
The water is typically heated either by an immersion heater (electric elements) or by an external heat source such as a boiler, which could be found in a kitchen.
The system relies entirely on the height of the header tank above the outlet point to create pressure. This means that the physical height difference between the tank and the water being emitted from the shower head dictates the flow rate of the water. The average pressure with a 1 metre head of water (difference) will produce 0.1 bar of water out of your shower.
The primary advantages of gravity fed water systems are that they are cheaper system to install and tend to be the most versatile, as you can add a booster pump to this system if needed. On the other hand, the lack of pressure can be an issue without a pump (generally known as a "low pressure" system in such cases).
This supplies cold water directly from the incoming cold water main. Mains fed water systems are only effective if you have a good incoming mains pressure to the house/building. In the past, water byelaws prohibited installing any sort of pump on the mains water. However, thanks to modern regulations, you are now able to fit certain types of pump - such as the Salamander HomeBoost - which do a fantastic job of boosting the pressure for you on demand.
Advantages are that this is a higher pressure system than gravity fed alternatives. Most modern mixer taps are designed for a higher pressure, so they would work correctly on a mains fed system. Additionally, you wouldn't need header tanks in the loft, so therefore giving you extra space.
Conversely, the main drawback is that mains pressure systems can fall prey to fluctuations in mains water pressure, both outside the home (high demand for water at the same time) and inside (opening a tap or flushing the toilet).
With a mains fed system you would need to heat the water. The most common methods are:
Disclaimer: Obviously this is just an overview. We would suggest obtaining specialist advice from a plumber or heating engineer before deciding on any type of system for your home.