What is the difference between stress and a stressor?

Stress is a state of mental or emotional strain or tension resulting from adverse or very demanding circumstances, whereas stressors are the factors that causes stress.
Stress, in psychology, is a specific response by the body to a stimulus, as fear or pain, that disturbs or interferes with the normal physiological equilibrium of an organism.
Let’s think extremes. Doesn’t mean that you get loads of work to do, you’re on a stress.
But when any of the stressors (an activity, event, or other stimulus that causes stress) is constantly disturbing you, then that’s the time your body is not in its normal physiological balance.
Say for EXAMPLE:
You were so close and attached to your mother, but suddenly she was killed by somebody. That moment (stressor) keeps bugging your head and therefore is giving you stress (your condition). When you encountered that stress, you can’t sleep well, you are always on a plan for a revenge, and so on, that it ruins your daily routine and daily lifestyle.
I repeat that is only an example. 🙂
Hope that helps.