One definition I've seen is that extroverts get energy from being with people, while introverts need to be alone to recharge.
I have in the past used precisely the latter to describe myself to others. Last Saturday I was at a student party: I spent the late afternoon and early evening chatting with both friends and new acquaintances pretty much non-stop, drank up to the point where my behaviour became just a bit more impulsive than usual, danced into the wee hours of the morning and exchanged some crude sexual rites with a stranger.
I unquestionably had fun. It will also take me at least a couple of weeks before I can talk to more than one people at a time without feeling exhausted. Until then, my free time will be spent with a pair of headphones, my reading queue, a 4X videogame, and no human interaction closer than Skype.
Followup to: Do you have High-Functioning Asperger's Syndrome?
LW reader Madbadger uses the metaphor of a GPU and a CPU in a desktop system to think about people with Asperger's Syndrome: general intelligence is like a CPU, being universal but only mediocre at any particular task, whereas the "social coprocessor" brainware in a Neurotypical brain is like a GPU: highly specialized but great at what it does. Neurotypical people are like computers with measly Pentium IV processors, but expensive Radeon HD 4890 GPUs. A High-functioning AS person is an Intel Core i7 Extreme Edition - with on-board graphics!
This analogy also covers the spectrum view of social/empathic abilities, you can think about having a weaker social coprocessor than average if you have some of the tendencies of AS but not others. You can even think of your score on the AQ Test as being like the Tom's Hardware Rating of your Coprocessor. (Lower numbers are better!).
If you lack that powerful social coprocessor, what can you do? Well, you'll have to run your social interactions "in software", i.e. explicitly reason through the complex human social game that most people play without ever really understanding. There are several tricks that a High-functioning AS person can use in this situation: