There is a big difference between shyness and social anxiety disorder, also known as social phobia. Shyness is a normal personality trait, so having shyness comes from people who do not necessarily have social anxiety disorder (which is an anxiety disorder). People who have shyness do not talk alot to people in social settings, and may avoid some social situations. However, people with shyness continue to live a normal life, and can overcome their shyness when they have important job and relationship obligations.
This is in marked contrast to social anxiety disorder, where the person avoids all social situations at all costs, to the point where it is highly disruptive to their life, so the social anxiety disrupts their job (they avoid as much as they can), their relationships, and their overall fulfillment from life.
So there is a marked difference betwee shyness and social anxiety disorder, where shyness is just a nuisance and does not disrupt your life, while social anxiety is highly disruptive to your life.