Well, people think that greenhouse gases (ozone and smoke and such) are slowly heating up the earth, but if you look at an average for the world's temperature for as long as it goes back, we are cooling down... The Earth's molten core can't stay hot forever... much like a chicken pot pie. Our core will eventually be solid rock (scientists think).
The earth's average temperature is rising slightly. Honestly the earth has done this before- gets really warm, everything melts, then everything freezes, then an ice age, and then repeat, it just takes an extremely long amount if timeto go thru this process
Atmospheric Pollution means all the chemicals, gases and poisons that we humans spread out into the air. Factory smoke stacks, car exhaust and the like are all examples of Atmospheric Pollution.
Global Warming refers to the heat that the Atmospheric Pollutants keep in the atmosphere of the planet often referred to as "Greenhouse Gases" where as the pollutants are letting the suns heat in but not back out like a greenhouse does, this in turn over years and centuries causes the planet to get warmer and warmer and if you believe what the Environmentalists are saying it'll one day become so hot that the Earth will no longer be able to sustain life of any sort.