Apparently there is a fundamental difference between the Final Five and the rest of the Humanoid Cylons. Extracted from the Cylon's Wikipedia Article:
Ronald D. Moore confirmed in an interview that the four are indeed fully Cylon — although "fundamentally different" from the others. Moore has also stated that unlike the other Humanoid Cylon models, the Final Five do not have model numbers. This is because in their original lives they were born to Cylon parents through sexual reproduction, rather than being built, as stated in "No Exit".
So, how different are from each other? Also, as stated in the previous quote, if they are born, not created, what makes them Cylons?