That is an easy one. A stack vent is the uppermost section of a main sewer stack from the top of the tee where the fixture ties in, to the roof. A vent stack is a vertical section of vent that picks up vents from mutilple floors and connects them to the stack vent, or through the roof. A main stack, is a drain or soil pipe that runs up through multiple floors at least 8 feet in height. A plumbing vent is a pipe that keeps a neutral air pressure in the pluming system so there is no negative or positive pressure when sewage flows into the drainage system.