Introvert or socially anxious?

This is a very good question!

I would say I am an extrovert who has social anxiety, which confuses many people when I try to explain it, and it actually makes a lot of people disbelief I even have social anxiety.

I really liked this description for Introvert vs. a shy person:

"An introvert and a shy person might be standing against the wall at a party, but the introvert prefers to be there, while the shy individual feels she has no choice." [Source].

Due to having social anxiety, I cannot be in social situations as much as I would like to nor can I enjoy them as much as I wish to. That's one way I distinguish it.

An introvert prefers deep connections with smaller groups of people. We tend to feel drained in large parties etc. and have to 'recharge' by ourselves for a while afterwards ( with a really good book, tea, netflix, etc.).

Socially anxiety can happen to both introverts and extroverts. It can feel just like @QuickJazz said. Like you don't have a choice but to be on the sidelines. In social situations you could feel like the odd person out, like your opinions don't really matter, or that your are cornered. It's different I think for everyone.

Everyone struggles with social anxiety from time to time. But anxiety turns into anxiety disorder when it's prolonged and it starts preventing you from leaving your home, going to the store to get groceries for yourself and/or your family, going to work everyday or school, etc.