A doctor tells the patient he has lung cancer. If, a few hours later, he:
1. Remembers having heard the news about his health from the doctor but doesn't believe he has cancer, that's denial.
2. Remembers having heard the news from the doctor, but believes he watched the doctor giving the news to someone else about his/her health, that's dissociation.
3. Doesn't remember hearing any news (or even meeting the doctor), that's repression.
That's the simple way to put it. You could look up the definitions of the terms in any standard textbook for a better understanding. Dissociation, particularly, is a very broad term.