Definitions are available at Wikipedia for need and want. But we are discussing more than that.
Need that is important for living or better to say which is necessary for survival. While wants are what you think you should have but it is something that you can live without. Desire is what you want from a long time, it may not change as frequently as wants. Need is what never change, you need it today and you need it tomorrow.
Drinking water, toothpaste, fruits, vegetables, pencil to write are your needs. Eating chocolate is wants. You have a strong desire to become a painter, and you may doodle sometimes. What you want will keep changing in a shorter period. But desire may not. Desire is stronger than want.
Priorities Need and Want
First you need to take care of what you need. What you want, you may control and delay. A strong desire can become a purpose but you need to compare your desire so you can priorities. But there is still a difference in desire and purpose. Purpose clarify your existence, who you are and what you do? Now Goals are aligned as per your purpose and are measurable so you can achieve and pursue your purpose.
One thing where people got confused, the purpose is not self-centric and nor it is materialistic. For example, a purpose for a teacher could be to teach or educate with passion. While a goal is to teach 2 hours daily. Finding the purpose of Life is nearly a duty of mankind. And I would say psychologically, finding your purpose is your need as well. Though you can live without it for the short term. But in the long term, it makes your existence more profound.
If you work on needs, chances you will achieve quickly. It gives you satisfaction. After that, if you work for wants, the chances of achieving is less. But if you define and control your wants, you can achieve your wants as well.
If you ignore your needs, you may face physical issues. If you ignore your wants without planning you may face emotional pain sometimes. You can always predict what you need but you can never predict your wants.
Why some people able to live peacefully with what they have? It is good that people learn something when they are kids. In childhood, parents play an important role in defining what we need, want and desire. The purpose is what we find ourself most of the time. And we keep refining our purpose of life as we pursue it.
And never think that you cannot learn these things in old age. Because Neuroplasticity tells you can learn something at any age. It is just some people practice control and meditation. And due to Increasing struggle of thoughts and practice of control, they build great habits of controlling their wants and defining their purpose. But you need to make sure, you will be happy on the journey as if you are happy and relax then you will learn and improvise easily.