Introverts gather strenght and energy when we spend time quietly by ourselves. We process things internally and usually spend a great deal of time ruminating on a situation--especially a new one--before jumping in. Very rarely will you see an introvert plunge in feet first to anything; we prefer to sit and think, and think, and think, and then wade into the pool. Of course there are other factors that lead to the decision of when and how to jump in (if we someone in need or in trouble, we often do jump in, we're not callous, just thoughtful). But most of the time,
even when we look like we're jumping in, usually we've given the matter a great deal of consideration--you just didn't realize that because all of that consideration happens silently inside our heads.
Extroverts, on the other hand, gather strenght and energy when they spend time around other people. They process things externally, by talking them out with friends and colleagues, and while they might spend just as much time as an introvert in deciding how and when to dive into the pool, everybody around them knows it. Their friends have heard the pros and cons and have been a necessary sounding board for the extrovert, because that's how extroverts process. That isn't to say that extroverts can't and don't keep secrets or that they're necessarily the chatty, out going one at a party. This is about how different types of people process thought and recharge their emotional and spiritual batteries--whether they recharge in moments of quiet solitude or by going out and hanging with a bunch of friends.
Realistically, very few people fall at either extreme of the continuum, but rather somewhere in the middle--although I'm a pretty extreme introvert. Depending on the test (more about those in a moment), I fall somewhere between 70%-100% pure introvert. Knowing this has allowed me to make time for myself without feeling guilty about it.
For anyone interested in taking a free version of the Myers Briggs test online, here are a few sites that offer it. The best results will obviously be garnered from taking the test with a human being to help you understand the results, but the free versions can give you some pretty awesome insight.