A lot of research has been conducted into personality traits since the 1960s. The current consensus is that there are at least five major traits, with many candidates for subsidiary and variant classifications. The " Big 5" are:

● Openness to experience (Closedness to experiences)

● Conscientiousness (Lack of conscientiousness)

● Extraversion (Introversion)

● Agreeableness (Disagreeableness)

● Neuroticism (Emotional Stability)

Or "OCEAN", if you like acronyms.  These are measured by (amongst others) the NEO-PI, or Neuroticism-Extroversion-Openness Personality Inventory.  Traits measured by the NEO Personality Inventory seem to be stable over time: as the results are consistent and replicable, and have been widely tested.  The model has good scientific support. 

The five factors were derived essentially through statistical analysis, based on much longer lists of the words people use to describe each other.  It's possible to see which terms correlate most strongly and are therefore likely to be describing the same trait.  For example "sociable" and "extrovert" are often found together, and seem to describe similar things.

The result of that analysis is the a model which suggests that there are basically five main dimensions to personality, as set out above.  This in itself tells us nothing about how these dimensions arise or what neurology underpins them.  We also need to understand that saying there are five basic dimensions is in no way the same as saying there are only these five - just that these groups capture most of the main things we say about each other's personalities.  These traits reflect how people describe each other, rather than from any objective assessment of how people actually work.  This means that they reflect the traits that people notice - which are not necessarily the traits that have most impact or effect, or even all the traits that there matter.  They certainly don't represent all the personality factors that exist.  It's also worth bearing in mind that there is no index as to which is most significant, which (as with everything in this field) depends on the circumstances and context.

Other Psychometric tests

Other psychometric tools worth knowing about include the Belbin team roles model (see this external link), and Michael Kirton's Adaptor - Innovator (KAI) scale, which are both fairly well supported by empirical evidence.  Kirton's A-I scale ranks personalities on a scale from 32 to 160.  "Adaptors" have KAI scores below 96, and "Innovators" score above this.  Adaptors tend to be conformist, working within the rules, innovators tend to challenge them.  See here  for a good overview (external link).  A difference in KAI score of 20 can be a major barrier to communication.  More at his own site,  KAI Centre. Whilst KAI is useful, I have reservations over the proprietarily, "it's all copyright" approach taken here. 

Early ideas about intelligence (Stanford-Binet) had only one dimension.  Personality tests now recognise that there are many thinking modes we can use, and that people have different default styles.  It's a wonderful, diverse world!  More on this here.