Hair cut, why do some hairdressers use razor and some scissors?

I have thick hair and my new hairdresser uses a razor. Why? I feel regular scissors plus those scissors with a serrated edge give me a better cut and makes my hair less heavy and prone to collaspe. Why does he use a razor? What is the difference between an razor cut and a scissor cut?Hair cut, why do some hairdressers use razor and some scissors?
A scissor makes a perfect cut. A razor makes your hair layered.