A person can be introvert if he/she is self conscious, afraid to express feelings in public, has hidden fears or could be introvert due to host of other similar reasons. A person in this category would be called shy.
But a person can also be introvert because that person is internally strong, self confident, feels no need to do things for social acceptance and truly does not find joy in mixing too much with others and so just enjoys being alone. These are the characteristics of a ‘reserved’ person.
A shy person is often left with feelings of inner frustration, lacks self confidence and is most likely unable to perform at his/her best. In short, the shyness is probably holding the person back in experiencing true contentment and joy in life.
Good news is that shyness can be overcome (at least partially), if one is determined and ready to work on it. Not so good news is that it can be hard work. But again good news is that any progress one makes will more than compensate the effort.
Further if it is a child who is introvert, then it falls upon the parents to find the reason and if the reason for being introvert is shyness, then they need to do something about it and help the child overcome these feelings.