Psychologists claim to use the Big 5 for reasons more objective than ascribed to them in lakeonfire's writeup. They claim that factor analysis, a statistical method to determine correlation, detected these five trait clusters as being strongly internally correlated and not strongly correlated with one another. Each of the Big 5 is thus actually a set of smaller traits, called facets, that are statistically linked.
The six facets of each of the big five are as follows: