Definitive means something is so clear, correct, accurate, or complete that no one could disagree and no one could add to it. E.g., "This book is regarded as the definitive account of the Iraq war." This means that everyone agrees that the book perfectly conveys the experience of the war and no book could ever be better. You can't replace "definitive" with "definite" in this phrase.
Definitive is used very often in expressions such as the definitive account of (for a movie, book, film, or news article about a person or event) or the definitive study of (for a research paper, academic report, etc.).
Definite just means someone is very sure or something is very likely to happen. A definite answer, definite plans, etc.