Passivhaus is a standard for building construction, including homes - it's a German standard, hence Passivhaus. The house is built to be extremely energy efficient, and has a comfortable interior climate maintained without a traditional heating system - or active cooling.
The passive house uses the sun, its occupants, household appliances and the extracted air - passive heat sources - to cover much of the heat demand. The remaining heat is provided by the supplied air, and is sufficient as a heat source because the house has such reduced heat losses.
To do this, a passive house is built with high levels of insulation; exceptional air tightness; uses passive solar gains - windows, walls and floors collect, store and distribute solar energy in winter and reject it in summer - as well as the internal heat sources; has energy-efficient glazing and frames; and has whole house mechanical ventilation with heat recovery for good indoor air quality. It should also have low energy appliances and lights.
The reduced energy demand of a passive house can be provided by renewable sources, such as solar panels and heat pumps.
You can open the windows of a passive house - although you shouldn't need to as the ventilation system provides fresh air.