There are 4 primary factors that alter climate (there are minor ones, but we're talking geological time scales here)
1. The sun. Earth gets about 1360 watts per square meter of energy from the sun. In contrast it gets about 20,000 times less energy from the core at the surface, less than .05 watts. It gets a similar negligible amount from space, such as distant stars, supernovae, or solar system objects. The sun has been slowly heating up over the 4.6 billion year lifespan of the solar system because, as hydrogen fuses into helium the core becomes denser and therefore hotter.
2. Position of the continents. 2 factors must exist in order to create an ice age (they are very rare in Earth's geological history). First, a continent must exist over one of the poles. North or south, doesn't matter. Second, tropical currents must be diverted preventing them from warming the polar continent. Right now we have both, Antarctica lies on the southern pole, and warm tropical water is diverted by the joining of North and South America at the Isthmus of Panama preventing it from getting the power needed to warm the poles.
3. Carbon Dioxide. Carbon Dioxide has a warming affect on the planet, preventing some infrared heat from escaping into space. Without this warming blanket of CO2 our planet would be an iceball. Less of it in the atmosphere can trigger an ice age. More of it obviously has the opposite affect.
4. The Milankovich Cycle. This refers to the four aspects of Earth's orbit around the sun. I won't overly complicate it for you (although if you're interested there's a lot you can learn) but basically the angle of the Earth toward the sun and the eccentricity of its orbit are the primary drivers of short term climate change (in the geological sense, we're talking tens to hundreds of thousands of years here). Milankovich Cycles alone cannot cause or end an ice age, they are only responsible for the ice caps growth and retreat throughout the ice age. Called 'interglacials'
