The letters INFP refer to: Introvert (as opposed to Extrovert), iNtuitive (as opposed to Sensing), Feeling (as opposed to Thinking), and Perceiving (as opposed to Judging). In other words:
- I tend to prefer solitude and interactions with a few close friends to socializing with many people
- I'm more drawn to imagination and possibility than to the sensible and practical
- I tend to value emotion and personal impact over logic and principles
- I prefer the open-ended, unstructured, and unplanned over the closed, organized, and scheduled
Keirsey divides his 16 types into four temperaments: Idealists (NF), Rationals (NT), Guardians (SJ), and Artisans (SP). INFP's fall into the Idealist category. Keirsey describes Idealists as being obsessed with introspection--we are on a quest not only to discover one's true self but to become one's true self. As such, we are probably more likely than others to find pleasure and significance in this sort of personality test. Supposedly, the other three temperaments can understand each other rather well, but find Idealists puzzling. Aside from their introspective nature, Idealists usually also value integrity and compassion. Idealists often wind up with careers in creative writing, journalism, teaching, counseling, and religious work.
According to Keirsey notable INFP's include George Orwell, Princess Diana, and Aldous Huxley. Among fictional characters and historical figures, Joan of Arc and Sir Galahad exemplify the type. Some say Shakespeare was an INFP; at any rate, his character Hamlet certainly can be viewed that way.
If you are interested, you can take the official test yourself, although you'll need to pay to get the full results. If you don't want to pay, you can at least find out which of the four temperaments you belong to for free. Or, if you look around, you can probably find an unofficial free one that will sort you into one of the sixteen types.
More information can be found on Wikipedia (of course) or in Keirsey's book, Please Understand Me (there's also an updated version called Please Understand Me II).