Complex sentence: The word "suburb" mostly refers to a residential area, either existing as part of a city (as in Australia and New Zealand) or as a separate residential community within commuting distance of a city (as in the United States and Canada).
Split into simpler sentences: The word "suburb" mostly refers to a residential area. It may be a residential area of a city (as in Australia and New Zealand), or a separate residential community within commuting distance of a city (as in the United States and Canada).