Defining the Greenhouse Effect
The greenhouse effect is a natural phenomena in which the greenhouse gases in the atmosphere trap heat leading to an increased temperature. As mankind is increasing the amount of greenhouse gases in air, the greenhouse effect is intensified resulting in an unnatural, rapid rise in temperature changing the climate as we know it - anthropogenic (man-made) climate change.
Energy emitted by our sun comes in the form of ultraviolet, visible and near-infrared radiation of which a part radiates towards Earth. The part of this radiation, which is not reflected back to space, penetrates the atmosphere unimpeded and is absorbed by Earth’s surface. Earth itself, as all bodies, emits the same amount of radiation it absorbs - keeping it’s equilibrium temperature. As the Earth’s surface is colder than the suns', it emits a different type of radiation - a lower energy one with longer wavelengths - infrared radiation. Radiative active gases in the atmosphere - greenhouse gases - absorb this infrared radiation and emit it in all directions. Parts of it fall back on to Earth’s surface. Because of that, Earth receives twice as much radiation from the atmosphere as it does from the sun resulting in a higher temperature - this is referred to as the greenhouse effect.
Defining the Runaway Greenhouse Effect
When there is enough greenhouse gas in an atmosphere, the planet absorbs more energy from the sun than it can radiate back to space and the runaway greenhouse effect occurs. In this state the warmer the surface temperature gets, the more radiation is trapped and the faster the planet warms up. It gets warm so fast the natural system can not regulate the temperature.
On Earth the following would happen: the hotter it gets the more water evaporates and acts as a greenhouse gas in the atmosphere. Water vapor traps more radiation making Earth even hotter until all oceans are evaporated and the planet is uninhabitable. This is what happened on Venus where there are no more liquid oceans.