note that the root word for both is 'define'- which means to tell the limits of. If you define a word you tell the limits of its meaning, if you define an area you show its boundaries. so definite is defined, known, decided. " the number 3 has a definite value". if you know how something will end," the outcome is definite." definitive describes a happening or object that defines something, or completely shows its meaning or its limits. a definitive collection contains all works by an artist, a definitive moment in a relationship shows the essence of the entire relationship in that one episode.

Good question. Similar words with very different meanings. Definite is sure. Definitive is final. My favorite online dictionary reference. It has a very nice free app as well: http://dictionary.reference.com/ Hope it helps.