Depending on which test you take, it can take anywhere from ten to twenty minutes, and every answer weeds out who you really are. It sections people off into one of two different choices in four different categories, leaving a four-letter "personality" that describes who you are as a person. The letters list as follows:
Introversion (I), or Extroversion (E): This category describes how you fit in the world around you. While not the true definition in regards to Myers & Briggs specifically, I tend to use describe the difference as being where you get your energy from, or where you "charge your battery". If you feel relaxed and ready to go after sitting at home on the computer, you could be introverted. If you prepare for a study session by hanging out with friends immediately prior, you're more likely to be extroverted.
Sensing (S), or Intuition (N): This category involves how you typically receive information from the outside world. Somebody more attuned to sensing will receive information from the world around them, using their five senses, and then act accordingly. But somebody that focuses more on intuition will take that information, and try to think about what that information means, and form conclusions based on that.
Thinking (T), or Feeling (F): This category refers to how you form the decisions you have to make on a day to day basis. Thinking types usually use rational thoughts and principles to strategize how they will go about solving their problems. Feeling types, by contrast, try to consider how everybody involved will be affected by a situation. The difference here is, simply, logic versus emotion. That isn't to say that people of one type or the other don't consider the opposite factor, but in the end, a thinking or feeling type will generally come to conclusions based on what they are more prone towards.
Judging (J), or Perceiving (P): The last category is all about the environment in which you are most comfortable in. It is all about how you structure your world. To put it simply, judging personality types enjoy structure and organization, while perceiving types are more flexible and spontaneous. Judging characteristics will tend to formulate plans and make a schedule for things, while perceiving types will approach situations by confronting things one at a time.
So, when all four of these are put together, you're left with an incredibly detailed description of who you are as a person. I myself am "INTJ". I highly recommend taking one of these tests, because the summary of what every personality type is like is pretty spot on, and for the site I like, it is so detailed it can tell me typical career paths for my type and problems I will probably face along the way. If you take the test, you could very well learn something about yourself, and we all have twenty free minutes to spend learning about something that can impact your life.