Narcissistic Personality Disorder (NPD) is a diagnosable condition on the continuum of Personality Disorders. Few Narcissists are diagnosed, however: a person must either present themselves for treatment or be sectioned to ensure diagnosis. Narcissists are very clever, they generally avoid compulsory referral. Narcissists believe They Are Right & Everybody Else Is Wrong; they will never seek treatment as they find themselves perfectly fine!
Common misconceptions:
[1] You can't call someone NPD if they haven't a diagnosis. Diagnosis is unlikely for the reasons above. Even Narcissists agree it's reasonable to determine a personality type as NPD (see links in first post). If you are in a relationship with a Narc, you don't need to be a clinician to know.
[2] He's got Aspergers or BPD, or is just a bit shit at dealing with people. Really? The obvious question is: why would you want to devote your life to someone like this? On a more technical note: emerging evidence, via biopsychology, suggests that Asperger and NPD brains exhibit similar differences from the average brain. And BPD is the baseline for all other Personality Disorders.
[3] S/He isn't "bad", they're just very hurt. Yes, they are very hurt. The more insight you gain into this spectrum, the more pain you feel for the sufferer. If you have a Narc parent, it's almost a given that you've gained tremendous insight - it was a necessity for your survival. The important thing is, YOU are not the same or you wouldn't be reading this. And understanding does NOT mean you can fix it.
[4] S/He just needs loving care. Yes, they do. They need it like a junkie needs heroin. You can keep on giving, they'll keep on taking and it will never be enough to satisfy them. Never. They'll suck you dry (emotional 'vampire') and then they'll rip you to shreds, just because you 'ran dry'.
[5] I get that s/he's abusive, but why say they've got a Personality Disorder? There's a spectrum of disorders: some are more 'needy'; some more 'domineering'; some are just fucking strange. At bottom, they're no more or less than deviations from the average. Nobody would choose a disordered realtionship unless they suffer from a need to be their partner's therapist. If this is you, try reading some of the links below.