It is the overall rise in the average temperature of the earth.
It occurs due to the increase in carbon dioxide which traps the sunlight inside the atmoshphere (also known as greenhouse effect).
The release of CFCs also causes the depletion of the ozone layer that intensifies the green house effect.
Global warming causing an ice age is a debatable argument. However, there is a strong argument that supports the theory:
The polar vortex is a low-pressure area in the polar regions that sends cold winds southwards i.e towards the equator, during the winters.
These cold winds are reflected back by jet winds that blow at a very high speed of about 275mph.
These jet winds prevent the areas between the tropic of cancer and the north pole and between the tropic of Capricorn and south pole, from harsh polar storms.
But due to global warming these jet winds, which generally blow around 30N and 30S, have shifted slightly towards the equator.
This is causing really harsh winters in the areas between the equator and tropical zones.