It is, only limited view, as you move, data gets sent to you.
For Example.
Lets say a map is 400 square units, you as a client are only allowed to know 20 square units, because the server does not send any information on anything past 20 square units out, as you move, you "explore" the locations, and monsters get into your 20 square unit view and you see them.
That's why you don't see maphacks that show all the monsters on the map, it simply isn't possible the way GGG does things. Pushedx can elaborate more.