File sharing between users

What is the "Wheel" user?

Does this user need to have RW access to a file in order for that file to be shared between users on the same MAC?

I set the file permissions to RW for each user but they remained read only for all users except the user who created the file.

Then I change the "Wheel" permission to read write and then the file became write enabled for the other users.

In the same direction ... what is the point of having a drop box if you can only write files to it but it is not possible to read any files from it.