What are the two pipes coming out of the back of my outside air conditioner?
A/C Refrigerant Line Set

Those are the refrigerant lines (what I would call Freon lines) that feed the condenser (the thing outside; half of a split system. The other half is the coil inside your furnace's plenum chamber). 

The big one (cold) is the suction line. The small one is the liquid line (hot; no need for insulation). Both of these lines run all the way back to your coil.

Picture