VMK stands for Virtual Magic Kingdom. From what I played through when it was released, it's basically meant to be an online representation of Walt Disney World/Land, with some things added in for gameplay purposes.
It was pretty fun, or at least, it met my expectations of fun when I was around 8-9 years old.
In terms of gameplay, here is some videos I pulled off Youtube that show what you could do in the game. Keep in mind, the footage was recorded a few hours before the game shut down:
https://www.youtube.com/watch?v=wjRONJH953oLooking back on it, I cringe at its appearances, but like most games I played back then, I can't deny that it gave me alot of fun when coming home from school.