This link gives excellent descriptions, with pictures
http://en.wikipedia.org/wiki/Maisonette
But basically a flat is part of a much larger building, on a single storey, with other flats above, below and alongside.
Tenement is a Scottish word for a block of flats.
A maisonette is like a flat, but on two storeys, with other maisonettes above, below and alongside.
A house is a home that stands on its ground, under its own roof, with no properties above or below. There may be other houses either side.