No matter how educated or conversant you are, you'll always meet with a person who can't fully understand the difference between Windows, Mac or Linux no matter how sufficient you have thrown the information at him/her.
But Phil Shapiro from PC World has made it easy with this simple explanation through fun conversation, check this out:
A middle school student I know asked me to explain the difference between Vista, XP, Linux and Mac OS.
Me: "You want to know the difference in real simple terms? Okay. Vista is bad gnews and GNU is good gnews. XP ain't such bad gnews, until you reach the blue screen of death, which ain't good gnews at all."
Middle schooler: "Okay, I think I understand. How about Mac OS?"
Me: "Mac OS is like a beautiful Bermuda island with a prison wall built all around it."
Middle schooler: "Really? How so?"
Me: "Well, Mac OS uses Digital Recreation Management (DRM), so if you want to build a sandcastle, you are forced to use an approved pail."
Middle schooler: "An approved pail? I want to use my own pail. That sounds to me like it's beyond the pale."
Me: "That indeed it is. It's beyond the pale. To use an approved pail is going to cost you $40 up front. Would you like to use Mastercard or VISA?"
Middle schooler: "I don't want to use Mastercard or VISA. I just want to use my own pail."
Me: "It sounds to me like you're ready to break out of prison and live a life with no bad gnews."