A maisonette is a flat or apartment that sits on two levels (most commonly ground floor and first floor) with internal stairs (or a fireman's pole for the Austin Powers' of this world) and will typically have its own front door on the ground floor level.
The word "maisonette" comes from the French word meaning "small house" and the word is typically used only in the UK (or former British colonies). A number of large Victorian and Edwardian properties in the UK will contain a lodgers unit with it's own entrance at the side or rear of the house. As these properties are generally separate units and self contained, they will typical have their own gas and electricity meters.