If you have a boiler and an airing cupboard containing a hot water cylinder that looks a bit like the one to the left, then you have a conventional boiler.
This type of boiler heats water and stores it in the cylinder in your airing cupboard, ready for when you need it. This means that it is possible to run out of hot water (for instance if the shower is being used a lot). It also means that you have an immersion heater. This is an electric heater built into the cylinder that can give you a boost of hot water if you run out, or if your boiler is not working. This should only be used as a back-up system and run for short periods.