Passive simply means that the unit does not emit IR light, but does detect it. All warm things emit some degree of IR. a PIR motion detector senses the difference between a warm body and the cold background.

Active IR emits IR light to illuminate an area to be "seen" with it's CCD that is sensitive to IR. Most new Sony camcorders use active IR for "night vision".

IR alone, with no modifier, simply refers to the spectrum of light known as Infra Red, which is slightly above human's vision range.

Dave