This article is from the Data Communications Cabling FAQ, by Peter Macaulay email@example.com with numerous contributions by others.
Communications Cable: primarily for telephone cable
Class 2 Cable: signaling cable primarily for data communications
Riser: vertical shaft used to route cable between floors
Plenum: Heating, Ventilation, Air Conditioning (HVAC) air return
area -- mostly drop ceilings. Also below raised floors
(where the underfloor area is used for ventilation).