Why is Earth called the "goldilocks" planet and how does Earth benefit from the greenhouse effect?

Earth is located in the "Goldilocks" zone around the Sun. This is the name given to the area where conditions are "just right" for the basic compounds that our form of life requires to exist (e.g. water, oxygen, nitrogen). If Earth had no greenhouse effect at all, then the night side of the planet would cool down extremely rapidly and probably freeze over.

On Venus, there is a runaway greenhouse effect. Pretty much all the heat is trapped, causing the temperature on both the day and the night side to reach around 470°C.

Earth is referred to as the "Goldilocks" planet because it is the only planet in our solar system that is "just right" for human inhabitation.  Goldilocks, from the children's story The Three Bears said that Baby Bears, chair, bed, and poridge temperature where "just right" so that is where the name comes from.

The Earth receives light from the Sun, and the amount of light that it receives is related to the distance from the Sun.  Mercury is going to be hotter than Mars.  Light hits a planet and becomes heat.

But the Moon is at the same distance from the Sun, and the Moon won't support life.  On the sunny side of the Moon, the temperature of rocks is over 200F, and on the dark side, around -200F.  That's not conducive to life.  

But the Earth has an atmosphere, and that atmosphere is transparent to visible light from the Sun but opaque to infra-red, or heat.  The visible light from the Sun can pass through the atmosphere and be absorbed by the ground.  The ground then radiates heat back into the atmosphere, and that heat can't get out because the atmosphere traps it.  This stabilizes the temperature of Earth around the freezing point of water and makes life possible.