Stress is comes in two main forms, good stress and bad stress, good stress can keep you motivated and concentrated, its helpful when you have an important task to do or goal to achieve and keeps you going.
Bad stress normally happens when you have no control or think you have no choice, like when your stuck in traffic on the way to an important appointment and as the time drags on you get even more stressed, you have no control and feel helpless, the more time ticks by the worse the feeling gets.
When we feel stressed it effects the hole body and mind, the physical body becomes stressed and the mind becomes clogged with anxiety and making decisions becomes harder.
Short stressful events do no real harm but long term stress reduces the body's ability to fight infection and process nutrients, it also make us more prone to becoming depressed or developing other mental health issues in the long term.
But we can change the risks by eating healthy and learning to relax, a good way to help stress is by leaning to mediate or start up some exercise, take up a hobby or just get out more and have some fun

The most important way to reduce stress is by changing the way we think, if you have low self esteem or find it hard to say no to people, that will heighten your risk of developing high levels of stress and anxiety.
A major problem with long term stress is we get used to it, we can think its just a normal part of life and put up with it until we snap or brake under strain.
If you find your drinking or smoking more to feel relaxed its a good sign that you need to take a look at how you live life and see what changes can be made to improve your health.