Copyright 2018, Michael Bryant
Loads of confusion here. Computers have nothing on central heating when it comes to unexplained jargon.
Here are four general types of boiler you're likely to hear mentioned by heating engineers:
Condensing boilers are a new(ish) type of boiler that extract more of the heat energy in the gas (or oil, or any other fuel) than non-condensing boilers and turn it into useable heat to warm your home with. This means they burn less gas for the same amount of heating, leading to slightly lower fuel bills and slightly less carbon dioxide emitted by the boiler into the atmosphere. Carbon dioxide is generally acknowledged to be a 'greenhouse gas', and is widely believed to contribute to 'global warming'. Condensing boilers have now been made compulsory by the Building Regulations (with a few limited exceptions) when you replace a domestic central heating boiler.
Combi boilers are often confused with condensing boilers but the expressions are completely unrelated. You do NOT have to fit a combi boiler under the Building Regulations, but you DO have to fit a condensing boiler.
Combi boilers heat the hot tap water as it is used. When a hot tap is turned on, the tap water flows directly through the boiler, the gas flames light and heat the water on it's way to the hot tap. In contrast, non-combi boilers heat a tank of hot-tap water and store it ready for use later.
Installers often recommend combi boilers because they are quicker, cheaper and easier to fit than non-combis, mainly because here are no tanks to supply and fit in the loft or airing cupboard. It is often not mentioned that they are also more complex and prone to breakdown than non-combi boilers.
The expression 'Regular Boiler' just means a non-condensing boiler. The expression was not needed until recently as (almost) all boilers were non-condensing.
System boilers are boilers designed to make the installer's life easier. They have an expansion vessel and the circulating pump built into them, saving the installer fitting these components separately.