The major difference between shy people and non-shy people is:

Introverts often have a small group of close friends. To those who know them, they’re often great listeners, can give thoughtful advice, and are extremely empathetic. A person who is severely shy may find it difficult to form close friendships like that, and can remain feeling awkward even around family and people they have known for their entire lives. An introvert can be fine with someone approaching them and beginning a conversation, but to someone who’s shy, that can be just as terrifying as the thought of starting a conversation themselves.

A big difference between the two is how the person feels about their lack of social interaction or desire for constant companionship. An introvert is fine with it: They really, really are. To an introvert, it’s not a problem to be alone with their thoughts or to have a quiet dinner with one or two close friends. For someone who’s shy, though, they tend to wish that it just wasn’t the case. They are the ones that long for a big group of friends, but just can’t bring themselves to reach out and put themselves into that situation.