How do you put the answer to this question into words? That's what I need help with. My sister is planning her family's first trip to the world and she has no idea what it's all about. She asked me, and I quote, "What is Disney World?" How in the world do you answer that question. I kind of found myself at a loss for words. I would love to hear how you would sum up what Disney World is to you.
I just went through this last year, when we vacationed at Disney with our best friends and their 4 children. None of them had ever been before.
There are 2 answers to this question... the technical descriptive answer, and one of sentiment.
As for the MAGICAL SPIRIT of Disney... once they arrive, they will either "get it" or they won't. No explaining is necessary for those who do, and no amount of explaining will change it for those who don't. Traveling as a kid with my parents, my Dad and I "got it." While my mother enjoyed our trips and could appreciate many things about Disney... how well it runs as it's own city, the cleaniness, the details, etc., I don't think she ever caught the spirit of Disney.
We spent many dinners with our friends, planning out our trip. Their questions were sometimes quite funny...
"But I thought you said we were staying at Disney, why do we need to ride a bus to the parks?" And,
"If there are highways running through Disney, isn't that kind of dangerous for the kids? 
I guess they were picturing their hotel was located inside an amusement park.

We discovered the key word for someone who has never been, is WORLD. Our friends could not grasp the sheer size of Disney World... so a
map of the world comes in handy!
I also planned trips for 2 other "first-time" families last year. All felt frustrated and overwhelmed on their first day... Dining Plans, Fast-Passes, Park Hopping, etc., it's a lot to learn. By the second day, they said everything I'd been telling them just "clicked!" They felt like pros.