The classic definition is "where do you get your energy from?".
I'm a classic introvert. I NEED time to myself. I just do. Now, I also happen to be outgoing, gregarious, high spirited, love people, talk to anyone... but that's purely because I'M NOT SHY. Shyness and introverted CAN go hand in hand, but don't necessarilly. Just like that shy "wallflower" constantly on the edge of the crowd, wanting to join in more than anything ISN'T introverted. They're a shy extrovert.
My son is a raging extrovert. He NEEDS to be around people. He starts going a little bonkers (tired, unhappy, cranky) when he's by himself too much. It took until AGE EIGHT before he could "play by himself" for 15 minutes. Even then... that was maybe once a day. Every 5 minutes popping in to "show you something funny/cool/interesting" doesn't count. When he is at his MOST calm/ relaxed/ happy is when he's surrounded by people. I predict he'll be a frat boy. And I rather strongly suspect that out of the two carreers I can really see him going for (cop or journalst) he'll pick cop. He NEEDS to be with other people, as much as possible, in order to be happy. He's the kid who, instaneously is "Mr. Popular". He likes (nearly) everyone, is really interested in what they have to say, think, are doing (and also if they're doing it "right", hence my suspicion of journalism or law... both are the two fields intent on whether other people are doing the 'right' thing). He's an extrovert AND outgoing.
For poor introverted little old me, he's fairly exhausting.
Most people are introverted. They LIKE being around other people, but they want their own space. ((Just like most people are followers)). True extroverts, and true leaders (my son is NOT a leader, the two don't necessarilly go hand in hand, either) are reeeeally rare statistically speaking.
Outgoing folks, otoh, whether introverted or extroverted are about 50/50 with shy folks whether the shy ones are introverted or extroverted.
Wheeee.... I got to put my behavioral psych to good use! (so rare!)