Larklight is the home in which the main protagonist Art (who recalls the adventures of the series from his perspective) and his family resides. It sits in an orbit around the moon and is supplied with comet-ice for its air supply.
It has no official top or bottom, as the gravity generator sits in the centre of the house and, much like a planet, has gravity drawing towards the centre from all sides.
We later discover that Larklight is Shaper Technology, hidden by Amelia Mumby (a Shaper and Arthur's mother) in the form of a house. It has the ability to travel through space faster than light, as well as, under certain conditions, travel through time (if a little inaccurately).
Larklight still retains these Shaper Technology until the end of the first book, when Sir Waverly Rain 'supposes' to destroy it, however, in the second book tthe parts are in the hotel 'Starcross'.