Fairytail land crosses over to the real world because of an evil queen's curse - the offical description from the web site:
About the Show
This is Fairy Tale Land. It's filled with magic, monsters and all of the characters we all know from stories growing up. It's real, and so are the people in it. But unlike the "happily ever after" you may have heard about, their stories continued, and The Evil Queen cast a Dark Curse over the land....
This is Storybrooke. It's a quiet, little, New England town filled with people who go about their everyday lives with no idea who they really are. The Queen's Curse has trapped them here and placed her in near-complete control. They have no real memories and no real hope.
Once Upon A Time is, at its core, a story about hope.
"For us, that’s what a fairy tale is. It’s that ability to think your life will get better. It’s why you buy a lottery ticket—because if you win you get to tell your boss that you’re quitting and you get to move to Paris or wherever and be who you always wanted to be. And that’s Cinderella, right? One day she’s sweeping up and the next she’s going to the ball. Adam and I just wanted to write about something hopeful that for one hour a week allows one to put everything aside and have that feeling that your dreams just may come true." - Edward Kitsis, Co-Creator/Executive Producer
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About the Show
This is Fairy Tale Land. It's filled with magic, monsters and all of the characters we all know from stories growing up. It's real, and so are the people in it. But unlike the "happily ever after" you may have heard about, their stories continued, and The Evil Queen cast a Dark Curse over the land....
This is Storybrooke. It's a quiet, little, New England town filled with people who go about their everyday lives with no idea who they really are. The Queen's Curse has trapped them here and placed her in near-complete control. They have no real memories and no real hope.
Once Upon A Time is, at its core, a story about hope.
"For us, that’s what a fairy tale is. It’s that ability to think your life will get better. It’s why you buy a lottery ticket—because if you win you get to tell your boss that you’re quitting and you get to move to Paris or wherever and be who you always wanted to be. And that’s Cinderella, right? One day she’s sweeping up and the next she’s going to the ball. Adam and I just wanted to write about something hopeful that for one hour a week allows one to put everything aside and have that feeling that your dreams just may come true."
- Edward Kitsis, Co-Creator/Executive Producer
http://beta.abc.go.com/shows/once-upon-a-time/about-the-show