Answer to Question 1
The runaway greenhouse effect is an unstable situation that occurs when an atmosphere changes its composition in a way that leads it to continue to change more and more rapidly. As an example, water vapor in the atmosphere is a greenhouse gas: its presence traps infrared radiation and increases the temperature of an atmosphere. Now if the amount of water vapor were to increase, perhaps by volcanic outgassing, then the temperature would increase. This would lead to enhanced evaporation of any surface water, which would further increase the temperature. Without other stabilizing effects, the temperature would increase until all the water was in the form of water vapor. Such a scenario probably occurred on Venus.
Answer to Question 2
E