The Earth and Venus are near each other in the Solar System, and are similar in size, density, and composition. Based on our understanding of the origin of the Solar System, we would expect that their initial atmospheres would have been rather similar. Yet the present atmospheres of the two planets could hardly be much more different than they are. How did this come to be? The reason is thought to lie in what is termed the "Runaway Greenhouse Effect".
Now, because of their molecular structures, certain gases like carbon dioxide and water vapor (and many others) have the property that they are essentially transparent to visible light but absorb IR radiation very strongly. Such compounds are sometimes termed greenhouse gases because, if they are present in a planetary atmosphere, they absorb the scattered IR radiation and tend to raise the temperature of the atmosphere by trapping solar energy. (The analogy with a real greenhouse is imperfect because the mechanism by which a greenhouse stays warm is different, but it is sufficiently good that the name "(Planetary) Greenhouse Effect" is now the common one for this phenomenon.)
Now suppose we increased the effectiveness of greenhouse heating of the Earth's atmosphere, for example by increasing the amount of solar radiation falling on it, or by increasing the concentration of greenhouse gases in the atmosphere (for example, by burning fossil fuels, which produce water vapor and carbon dioxide as byproducts of burning). We would then expect the temperature to rise in the atmosphere (assuming no other effects intervened---a big "if" in the realistic case since the atmosphere is complicated). This would be a greenhouse effect.
Thus, we believe that in the case of Venus the initial solar heating kept oceans from forming, or kept them from staying around if they did form, and the subsequent lack of rainfall and failure of plant life to evolve kept the carbon dioxide in the atmosphere rather than binding it in the rocks as is the case for the Earth; thus, Venus has an environmental disaster for an atmosphere.
The sobering warning for us is obvious: we have to be extremely concerned about processes such as burning of fossil fuels in large volumes that might (we don't know for sure because the scientific questions are complex) have the potential to trigger a runaway greenhouse effect and produce on the Earth atmospheric conditions such as those found on Venus.
When we talk about a "runaway effect" or a "positive feedback loop" we are talking about the concept of an "instability." The idea of an instability is that a slight perturbation leads to a growing perturbation and then the effect becomes completely unstable catastrophically. Other examples include knocking down a house of cards. You slightly perturb the equilibrium and the whole thing comes crashing down.
It turns out that the DYNAMO AMPLIFICATION of magnetic fields is also an instability.
(There are also emotional instabilities or emotional feedback loops: someone can get slightly depressed but then this can cause them to enter a psychological state where they do things that make them further depressed and then the effect amplifies).