What is the difference between social anxiety and introvert?

An introvert can be described as a person that is the "opposite" of a extrovert. This person doesn't have to be quiet and unapprochable at all..it's just a person who feels at ease and prefers it when they can get some alone time. Theres nothing cool or special with that, as many likes to imply for some reason.

An example; a extremely sociable person who frequently at times just doesn't bother talking with anyone at all to recharge can be a introvert. And vice versa, as there are quiet people who can be quite talkactive and HATES being alone and not socialising, they are still extroverts.

Social anxiety however, is what you could consider a mental "illness", in the way that it makes it impossible for you to function as you normally should do. A socially anxious person may find it hell to be in ANY type of social situation, because they show sympthoms of anxiety when they go through it. (they may start shaking, sweat a lot, blush, feel uneasy, stutter and etc). As a result they completly start avoiding any social situation that triggers theirs symptoms (avoiding school, avoiding speeches and public situations, etc) It's definitely not associated with anything positive, unlike introvertedness.

TDLR; one is a personality trait, the other one a illness.

Introvert usually means social interaction drains you of mental energy, as opposed to an extrovert that might feel mais energetic after social interaction.

Social anxiety has nothing to do with energy, it's the constant fear that people will judge you, murder you, attack you, ETC, even if that fear is completely irrational.