Introversion and extroversion are well defined in personality psychology. What it really comes down to is where you prefer and receive stimulation from the most. Broadly speaking extroverts tend to seek stimulation from the external environment, hence extro-vert. These people prefer to be at parties and around people because that is where most of the stimulation comes for them. Introverts tend to be more stimulated by internal environments, like thoughts and feelings. Parties and other people can provide a large amount of additional stimulation that introverts might have trouble dealing with because of the internal stimulation as well. Now this is very broadly speaking and no one person is one or the other completely. It is a spectrum and people fall somewhere inbetween the two. The fear you speak of, I would think, is more of a side effect then a cause of being an introvert. Just because someone may identify as an introvert does not mean they are scared of what people might say. Similarly just because someone is an extrovert does not mean that are not scared of what someone might say.