What's the difference between a combi boiler and a system boiler?

The distinction isn't quite as clear-cut as it's sometimes made out to be, but the main difference is that a combi boiler heats water on demand for both radiators and taps, while a system boiler needs a separate hot water cylinder to store heated water for your taps. Combis suit smaller homes without much space, whereas system boilers work better if you have multiple bathrooms needing hot water at the same time.