The stack effect is the movement of air into or out of a building. It is caused by buoyancy which occurs due to a difference in air density inside and outside of a building resulting from different temperatures and moisture levels. This causes a positive or negative buoyancy force. The greater the thermal difference and height of the building, the stronger the buoyancey force - or stack effect.
Image Source: EPA.gov
As modern windows and doors have become more airtight, the difference in temperature and moisture levels inside and outisde a building or home increases. This causes the stack effect to be most severe in attics that lack proper insulation. Air is pulled from living spaces and into the attic causing not only heating and cooling loss, but also a damp, overheated attic.
Source: Wikipedia