I come with psych babble
The Big Five model, or Five Factor Model, alterantively called the "OCEAN" model (as an acronym of the five thingies it entails) is apparently the preffered method of measuring personality nowadays. It views personality as consisting of 5 distinct dimensions: Openness to experience, Conscientiousness, Extraversion, Agreeableness and Neuroticism. Research psychologists apparently found these 5 clear factors upon analyzing a host of personality test questions and performing factor analysis to see how many and what factors the answers best load onto (statistical procedure that is evil and I cannot begin to explain...rather irrelevant here, fortunatelly.). They also claim that doing a lexical analysis of many languages, personality trait words somehow cluster in these five areas.
These dimensions are factors not types, so people dont fall into categories but vary continuously along each dimension.
The five dimensions are sometimes broken down into more specific subscales. Thus, for example, Neuroticism includes subscales of Anxiety, Anger, Depression, Self-Consciousness, Immoderation, Vulnerability...
The test works on the basis of comparing people to norms within the population.
Here are sites for more info:
http://www.personalityresearch.org/bigfive.html
http://www.centacs.com/quickstart.htm
Here you can take the test, called the IPIP-neo or its short version:
http://www.personalitytest.net/ipip/ipipneo1.htm
hope that helps... sorry if I wasnt thorough enough, its late and Im a bit tired