What is the difference between social anxiety disorder and shyness? It seems that there are similar symptoms shown by people who suffer social anxiety and shyness, but what is actually the main difference between them? Thank you.
It's a matter of degree.

First of all, "shyness" is not a psychological or psychiatric term, but an everyday English word denoting a commonly observable personality characteristic on a par with courage, cheerfulness, or honesty. The meaning of "shyness" is not exactly defined, and people may use the word "shyness" to refer to different kinds of behaviors, from being introverted (and not interested in social interaction) to being afraid of social interaction.

"Social anxiety disorder", on the other hand, is a psychiatric and clinical psychological term, the meaning of which is clearly and exactly defined by the scientific community. That means that while there is of course some debate about what social anxiety disorder is, there is agreement about what the term currently means.

As for the relation between shyness and social anxiety disorder, shyness is generally conceived as a mild from of social anxiety, which leaves the person able and functioning in his or her life, while social anxiety disorder is such a strong anxiety that the person suffering from it cannot partake in a normal way, is disabled by this anxiety, and suffers from its effect.