Combi Boiler. A relatively new type of heating system. You will normally find them in smaller properties with limited space. The benefit is thy require no feed tank or hot water tank. Hot water is made as you turn on the tap. All your water including the hot is mains fed so you have good pressure throughout.
You only need to turn off the mains stopcock to turn all the water off.
Advantages are that they are compact, require no hot water or feed tank, and you get mains pressure to all taps.
The disadvantage of these types are that flow rate can be limited so not suitable for multiple heavy demand (eg homes with several bathrooms.) locations