A Combi boiler heats water directly from the mains when you turn on a tap. So you get hot water instantly – without a cylinder or a tank in the loft.
If you have a hot water storage cylinder as well as a cold water tank in the loft, your boiler is likely to be Standard. These are also called Regular, Traditional or Conventional boilers.
If you have a hot water storage cylinder but no cold water tank in the loft, you're likely to have a System boiler. The boiler will normally have a pressure gauge on the front of it.
A back boiler unit (BBU) is a boiler built-in behind an electric or gas fireplace. You can also choose this option if you have a Potterton Powermax.