For example, I've taken the MBTI in several different formats, and I'm usually certain to come out as with consistent results in Attitude and Lifestyle preferences, but fluctuating in my Functions (Information-gathering and Decision-making); I am an I-something-something-J. Wikipedia's entry sums up the various attributes nicely, in case you're unfamiliar with them (or, like me, tend to get confused):
I recently took two different MBTI tests within a very short time period (this was the first one); I came out as ISFJ in one, ISTJ in the other. I'm inferring from this that the questions asked about information-gathering preferences must make a difference in whether I score higher as Thinking or Feeling - and honestly, I think that's accurate. In some situations I'm more objective, and in others I'm not. Sometimes I do come across as more logical, and sometimes more empathetic.
Functions (S-N and T-F)Sensing and Intuition are the information-gathering (perceiving) functions. They describe how new information is understood and interpreted. Individuals with a preference for sensing prefer to trust information that is in the present, tangible and concrete: that is, information that can be understood by the five senses. They tend to distrust hunches that seem to come out of nowhere. They prefer to look for detail and facts. For them, the meaning is in the data. On the other hand, those with a preference for intuition tend to trust information that is more abstract or theoretical, that can be associated with other information (either remembered or discovered by seeking a wider context or pattern). They may be more interested in future possibilities. They tend to trust those flashes of insight that seem to bubble up from the unconscious mind. The meaning is in how the data relates to the pattern or theory.
Thinking and Feeling are the decision-making (judging) functions. Both Thinking and Feeling types strive to make rational choices, based on the data received from their information-gathering functions (S or N). Those with a preference for Feeling prefer to come to decisions by associating or empathizing with the situation, looking at it 'from the inside' and weighing the situation to achieve, on balance, the greatest harmony, consensus and fit, considering the needs of the people involved. Those with a preference for Thinking prefer to decide things from a more detached standpoint, measuring the decision by what seems reasonable, logical, causal, consistent and matching a given set of rules.
People with a preference for Extraversion draw energy from action: they tend to act, then reflect, then act further. If they are inactive, their level of energy and motivation tends to decline. Conversely, those whose preference is Introversion become less energized as they act: they prefer to reflect, then act, then reflect again. People with Introversion preferences need time out to reflect in order to rebuild energy.
In addition to the two Function pairs and Attitudes, Myers and Briggs identified that individuals also had a preference to show either their Judging function (T or F) or their Perceiving function (S or N) when relating to the outside world. Myers and Briggs called this one's "ambassador," that is, the one sent forth to deal with the world.
Myers and Briggs taught that types ending in J show the world their Judging function - either T or F. So TJ types tend to appear to the world as logical, and FJ types as empathetic. According to Myers these types prefer to have matters settled.
Those types ending in P show the world their Perceiving function - either S or N. So SP types tend to appear to the world as concrete, and NP types as abstract. According to Myers, these types prefer to keep matters open.
There are sixteen possible combinations of MBTI attributes - in-depth descriptions can be found here - and, according to the Kiersey Sorter, the combination of Information-Gathering and Lifestyle attributes can be sorted into four Temperaments: SJ/Guardian (administrators/conservators - that's mine), SP/Artisan (entertainers/operators), NF/Idealist (advocates/mentors), and NT/Rational (engineers/coordinators). I think I'm probably in the right niche with this one, since those attributes seem to stay consistent.
The second MBTI test I took was this one, thanks to Bub and Pie's fascinating "Myers-Briggs Analysis of Harry Potter." She has concluded that the series is "all about the SJs and SPs" - unfortunately for me, I'm in the SJ camp with Dolores Umbridge, although she does find some more welcome associates for me there too, like Molly Weasley and Minerva McGonagall. However, she also offers a link to each MBTI type and an associated Harry Potter character; according to that, I'm either Hermione (ISTJ) or Hagrid (ISFJ), which seems like a rather curious mixture to me, but I can live with either one.
Knowing one's MBTI type can be useful in understanding how one deals with the world, but there are lots of other personality tests and classifiers out there. I found the Interaction Style Assessment Test via Penelope Trunk. It's similar to an MBTI test in that it asks questions about how you approach information, decision-making, and other people, and evaluates your responses to determine which of four Interaction Styles defines you best. (The link is to my own result - which I think is pretty accurate - but the page describes all four styles, and provides a useful grid to help identify where and how each works most effectively.)
So now I've learned I'm officially an IS-something-J Contemplator. And based on my own self-knowledge prior to taking these tests, I have no disagreement with any of those findings - they're pretty much confirmation. (The "I" was never in dispute, anyway.) Having said that, I'm not sure it would be a good thing to be taken by surprise by the results of your own personality tests - although I guess I hadn't realized Hagrid and I had quite so much in common.