Characters do not change. Opinions alter, but characters are only developed."
This is the theory that has spawned the empirical breed of 'personality tests', rather than the projective, open-ended tests which characterise psychology.
The trait perspective looks at personality as a cocktail of one-or-the-other behaviours. They try to explain a person's behaviour based on wherther they are outgoing or shy, trusting or paranoid, organised or unpredictable. Tests are used to see which of the two possibilities for each trait are preferred, and then gather that information to make a rather accurate explanation of the person's overall behaviour. For example, using the Meyers-Briggs test (a person is tested for traits - Introvert/Extrovert; Intuitive/Sensing, Feeling/Thinking, and Percieving/Judging. If a person has the results Extrovert/Intuitive/Thinking/Percieving, for example, they are inventive, have very active imaginations, have no sense of time, and start lots of projects - not necessarily finishing them, mind you.
Of course, the trait theory probably doesn't take into account one very important factor - setting. It has been proven that there are obvious changes in a person's personality when put in different situations. The most noticed changes are those between introversion and extroversion, and judging and percieving. One example is h2g2 - people who are normally somewhat shy and reserved may become outspoken. Another is the difference between a person at work and home - a person with a very messy home my keep their office neat.
The trait theory describes what is probably the closest to the actual definition of 'personality' in general - noticeable characteristics which makes a person who they are.