What is the difference between narcissism and having real confidence in yourself?

One is a disorder. Narcissistic personality disorder is by definition a disorder, meaning it has to cause signficant dysfunction in one's life, health, employment etc. It involves not so much confidence as self-love, meaning to be infatuated with who you are and one's accomplishments to the excusion fo all others. Self-confidence is more a healthy knowledge of one's own abilities and would be considered a good thing.
Narcissism. Narcissists will tell you they're the best at everything they undertake, bar none. Accurate self confidence implies feelings of being good enough at what one undertakes, but leaves space open for learning.