Narcissists are people with idealized visions of themselves; narcissists often make grandiose claims about themselves and tout their perceived positive traits and accomplishments. Narcissistic personality disorder is a specific condition from which some narcissists suffer.
When people use the term narcissist informally, they're often speaking about a person with an inflated sense of self-worth or a high degree of selfishness. Such people might not suffer from a mental illness, and their perceived narcissism might not lead to significant distress in their lives. There's no clear distinction between non-clinical narcissism and high general confidence.
When people use the term to describe someone with narcissistic personality disorder, they're referring to a specific mental condition characterized by an unrealistic sense of self-worth, excessively grandiose thinking, and a need for constant admiration, among other symptoms. Like other types of personality disorders, narcissistic personality disorder often entails a lack of empathy, and many people with the condition appear rude or condescending.
As with many personality disorders, people with narcissistic personality disorder generally don't think they need help. Narcissistic behavior, however, can lead to significant problems in life, particularly in relationships with other people, but even this distress might not be enough for sufferers to seek help. Treatment for narcissistic personality disorder generally consists of long-term psychotherapy, although medication can help with certain symptoms.