The answer to 'difference between single stage and two stage furnace' is that a single stage furnace has only one level of heating output, while a two stage furnace has two levels of heating output. A single stage furnace runs at full capacity whenever it's on, while a two stage furnace can run at a lower, more efficient level when the home doesn't need maximum heating.
The two stage furnace provides more precise temperature control and can save on energy costs, especially in milder weather when the lower heating stage is sufficient. It also runs more quietly than a single stage furnace. However, two stage furnaces are generally more expensive to purchase and install than single stage models.