The words "introvert" and "extrovert" have been used by a lot of people over the years, starting all the way back at Freud I think. If a modern scientist uses these words, though, they're probably talking about it in terms of the Big Five[1] personality model, which is mostly just identifying some observed clusters of traits with strong correlations that tend to persist over a person's life. People argue about whether there might be more emperical clusters worth adding, or whether they might be made more orthogonal but generally there's a good amount of rigor behind them, far more than IQ for instance.

Introversion/Extroversion is very much a matter of degree, just like pretty much any other human trait. It doesn't uniquely determine behavior by any means, humans are very complex creatures. However, there is a pretty strong correlation [2] between someone acting in an introverted or extroverted manner in one situation and acting the same way later.

[1]http://en.wikipedia.org/wiki/Big_Five_personality_traits

[2]http://en.wikipedia.org/wiki/Correlation

EDIT: All of which isn't to say that people can't use those terms in pseudo-scientific ways, or that the author of the article isn't. After all, people use terms like 'mass' and 'gravity' in pseudo scientific ways too.