I believe that psychopathy/sociopathy (more or less the same thing, but one term is used more in a medical context and the other in legal) is basically an inability to feel genuine emotion other than various shades of anger. Psychopaths are good at figuring out what makes people tick and manipulating them. They are egocentric and have no genuine regard for others, though they are talented at mimicking emotion--they get kicks out of deceiving people. Scans show that a psychopath's brain waves are completely different from those of a normal human. There's essentially no "cure" for psychopathy: it's somewhat effective to teach psychopaths that antisocial behavior isn't in their best interest, but there's really no way to change the way their brains work.

There's an official Psychopathy Checklist:
Factor1: Personality "Aggressive narcissism"
Glibness/superficial charm
Grandiose sense of self-worth
Pathological lying
Cunning/manipulative
Lack of remorse or guilt
Shallow affect
Callous/lack of empathy
Failure to accept responsibility for own actions

Factor2: Case history "Socially deviant lifestyle".
Need for stimulation/proneness to boredom
Parasitic lifestyle
Poor behavioral control
Promiscuous sexual behavior
Lack of realistic long-term goals
Impulsivity
Irresponsibility
Juvenile delinquency
Early behavior problems
Revocation of conditional release

Traits not correlated with either factor
Many short-term marital relationships
Criminal versatility