Chances are at some point in your life, either at work or at school, you've taken the Myers-Briggs Type Indicator. If you haven't taken it, someone might have described you as a "total INFJ" even though you have no idea what that actually means. Essentially, Myers-Briggs is a personality test based on how you perceive the world and make decisions. If you've never taken it, here's the free personality test.
The results can vary a ton person to person, but I'm going to break them down for you the best way I know how....using TV characters!! This will totally help you make sense of the different personality types out there, and figure out which you're most similar to!
Introvert (I) v. Extrovert (E)
This one is probably easiest to understand. Introverts are private, and prefer to observe rather than be center of attention. Extroverts, on the other hand, are out-going, talkative and like when people focus on them. Ted is a classic introvert, preferring to observe and take in information (maybe that's why he overthinks every romantic relationship). Then we've got Marshall, probably the friendliest guy on TV, who's warm, outgoing and a little zany.
Sensing (S) v. Intuition (N)
Sensers are practical, appreciating fact and reason, while those with Intuition are imaginative, seeing things big picture in a figurative, instead of literal way. Nick is a little more serious when it comes to thinking, while Jess uses her enthusiasm, passion and imagination. Teachers often have an I personality, so it fits Ms. Day perfectly.
Thinking (T) v. Feeling (F)
Thinkers are level-headed, preferring to use logic to make decisions, while Feelers tend to make decisions based on emotion and personal values. Ron Swanson, probably the most logical person in the world, would never make any decision based on emotion (probably because he has none). Andy Dwyer, however, has never really given anything much thought, only making decisions based on what he wants to do in the moment (i.e. turning a dinner party into a surprise wedding).
Perceiving (P) v. Judging (J)
Perceivers are spontaneous improvisers who can go with the flow. Judgers prefer rules and deadlines and must have plans figured out or they'll go crazy. This makes perfect sense with Joey and Ross. Joey is enthusiastic and simple. He can be a little impulsive, but he totally lives in the moment. Ross, on the other hand, over thinks every single situation. J's are actually known for being bad flirters, and we all know Ross is the world's most nervous flirter.
If you're still confused, check out this chart for a detailed explanation of personality types.