There are a lot of sources that can cause stress, and these can show substantial differences between different people. But most of these causes can be grouped into a distinction between three common causes of stress:

Important life changes: every big change in someone’s life causes stress, changing jobs, starting college, divorce, … . In general, it can be said that how bigger the change, how bigger the stress that accompanies it.
Daily irritations: not just big life events cause stress, the accumulation of small daily irritations do it as well. A machine that breaks, kids acting up, traffic jam, deadlines, and so on al cause stress. Research has shown that these small daily stress sources are just as important as big life changes in causing stress.
Perception of the person: it has been found that the effects of the previous stress sources are not the same on everyone. Their effects depend highly upon how a person judges these events. The perception of the relative importance of these events influences the susceptibility of a person to stress.

Consequences of Stress

The consequences of prolonged stress have been studied quite a bit. The most common ones are:

Physiological changes: stress elicits two physiological changes: an immediate shock reaction and a long-term adaptation. The shock reaction activates the ‘fight or flight’ reflex and the long-term adaptation is a counter-reaction to ensure that not all of the energy is used.
Immunity: stress decreases the working and efficiency of the immune system, which explains why people under a lot of stress are more prone to infections.
Psychological changes: exposure to stress increases the chance of experiencing negative emotions, such as fear, gloom, frustration and anger. The cognitive functioning is also influenced by stress. A little stress heightens cognitive functioning, but too much stress decreases it.
Behavioral changes: stress is visible in a person’s behavior. Tense face, shaky voice, shivering and acting scared are all signs exhibit in great stress.

Dealing with Stress

The extent of these effects depends on how people deal with stress. There are generally two main ways to do this:

Dealing with the stress factors: by direct intervention, one can try to change ones relationship the stress-causing event. This can be done by ‘approach’ (confronting what is causing the stress), or avoidance.
Dealing with the consequences of stress: in some cases, dealing directly with the stress factors is not possible and people have to limit themselves to controlling the ‘symptoms’. Techniques that can be used to achieve this are reinterpretation, changing perception and dealing with the physical symptoms through, for example, relaxation therapy.

References

Aldwin, C.M. (2007). Stress, Coping and Development. The Guilford Press.
Lupien, S.J.; McEwen, B.S.; Gunnar, M.R. & Heim, C. (2009). Effects of stress throughout the lifespan on the brain, behavior and cognition. Nature Reviews Neuroscience. 10, pp. 434 – 445.