This is the most traditional type of boiler which allows you to have stored hot water within a cylinder, The boiler heats your radiators and the cylinder Via an external pump and three port valve. Ideal for houses with two bathrooms. The downside is that your hot water pressure can be a bit slow.
This type of Boiler does your heating and hot water and is a good all rounder its ideal for flats and small houses with only one bathroom.
This is the most common in new houses and is hands down the most efficient and economical. it gives you great hot water pressure throughout your house, the boiler has the Pump and expansion vessel inside so looks tidy. It's ideal for houses with 2 or more bathrooms.