PVC (Polyvinyl Chloride) is what your standard Category 5e and Category 6 cable jacket is constructed of.  This PVC jacket when burning or smoldering releases hydrochloric acid and dioxin which are both toxic.  Plenum cable is made of Teflon or FEP which gives off much less poisonous gas than PVC when it burns.

So when do you have to use Plenum vs the PVC by code?
Basically this has to do with the construction of the building and how the HVAC system is designed.  If the return air to the A/C or heating system comes from the open space between floors, walls or from under the floor this area is considered a Plenum air space and Plenum cable by code needs to be used.If there are no ducts bringing air back to the A/C-Heat then you need Plenum cable.

Tip:  If you can speak with an inspector before you start the job, do it, as in some areas they require plenum even when the code would not require it.

According to Article 800 of the National Electrical Code (NEC), plenum cables must comply with the specifications for flammability and smoke density outlined in Underwriters Laboratories (UL) and National Fire Protection Association (NFPA) testing methods.

Plenum cable is designated by the CMP (Communications Plenum Rated Cable) marking.CMP cable can also be substituted for CMR or CM cable as well.