The Big Five are the five broad dimensions that keep appearing when large numbers of people rate themselves or others on long lists of trait words: openness to experience, conscientiousness, extraversion, agreeableness and neuroticism. They are descriptive rather than explanatory, continuous rather than types, and they have been recovered in many languages, which is why they are the standard framework in personality research.
Where they came from
The approach is lexical: if a trait matters, a language will have words for it, so the dictionary is the raw material. Researchers from the 1930s onward collected thousands of English trait adjectives, had people rate themselves and others on them, and factor-analysed the ratings. Five broad factors emerged again and again. Goldberg (1992) published a set of marker adjectives for them, and later wrote the public-domain statements, such as “Am the life of the party”, that the International Personality Item Pool holds and that the tests on this site use. Johnson (2014) built the 120-item inventory that measures each of the five with six facets.
What each measures
Openness is appetite for ideas, art, feelings and novelty. Conscientiousness is organisation, persistence and care. Extraversion is the pull towards company, activity and excitement. Agreeableness is the weight given to other people’s feelings and interests. Neuroticism is how readily negative feelings arise and how long they last; its other end is emotional stability. Each is a bundle of narrower facets, and two people with the same domain score can differ a lot on the facets, which is why the 120-item test scores thirty of them.
What it does not measure
The Big Five does not measure intelligence, values, interests, motives, mental health or skills. It is not a theory of why people are as they are. It does not sort anyone into a type, and it does not say what anyone should do. It describes tendencies as a person reports them, compared with other people.
What it means for your results
Read each trait as a position on a scale among people like those who took the test, and read the facets to see what drives the domain score. A result in the middle half of the sample on any trait means you are like most people on it. The trait pages say what each trait is, what high and low look like in daily life, and what the research finds, with the papers cited.