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.
The accepted answer is correct, it is basically a matter of degree, plus some other things.
Adding more details, refer to the following definition of Social Anxiety Disorder (previously known as Social Phobia) taken from The Diagnostic and Statistical Manual of Mental Disorders 5th edition:
A. A persistent fear of one or more social or performance situations in which the person is exposed to unfamiliar people or to possible scrutiny by others. The individual fears that he or she will act in a way (or show anxiety symptoms) that will be embarrassing and humiliating.
B. Exposure to the feared situation almost invariably provokes anxiety, which may take the form of a situationally bound or situationally pre-disposed Panic Attack.
C. The person recognizes that this fear is unreasonable or excessive.
D. The feared situations are avoided or else are endured with intense anxiety and distress.
E. The avoidance, anxious anticipation, or distress in the feared social or performance situation(s) interferes significantly with the person's normal routine, occupational (academic) functioning, or social activities or relationships, or there is marked distress about having the phobia.
F. The fear, anxiety, or avoidance is persistent, typically lasting 6 or more months.
G. The fear or avoidance is not due to direct physiological effects of a substance (e.g., drugs, medications) or a general medical condition not better accounted for by another mental disorder
Perhaps more specifically, it is written in this DSM-5 Social Anxiety Disorder Fact Sheet, that (emphasizes are from me):
Social anxiety disorder is about more than just shyness and can be considerably disabling. A diagnosis requires that a person’s fear or anxiety be out of proportion—in frequency and/or duration—to the actual situation. The symptoms must be persistent, lasting six months or longer ... To be diagnosed with social anxiety disorder, the person must suffer significant distress or impairment that interferes with his or her ordinary routine in social settings, at work or school, or during other everyday activities
It's also important to note that there's been a great deal of effort in defining abnormality (disorder). I don't own any version of DSM (I do have a physical copy of Clinical Psychology by Andrew M. Pomerantz and Abnormal Psychology by Nolen-Hoeksema, but they are currently borrowed by friends), but the criteria for abnormality usually involves (as seen here and here):
- Emotional distress
- Impaired functioning in day-to-day life
- Deviance from cultural norm
- Statistical infrequency
- Behavior that places people at risk for personal suffering, pain, disability, or death
And it usually has to last for a certain period of time before it can be diagnosed as a disorder.
DO NOTE, however, that no matter how much you feel that you or anyone you know seems to fit with one or more of the above criteria, you CAN'T simply conclude that he/she/they have a mental disorder. Diagnosis of mental disorder should only be done by someone who is certified by law to do so, such as psychologist or psychiatrist. And even then, it should be done only after complete, thorough, and proven analysis.