Q:

What happens to files across multiple file systems and OSs

How can it be that a file on an ext3 partion in Linux, which, for example, has different metadata, can get transferred to a Windows NTFS and we still can open and read it correctly? 
Don't you have to convert it somehow to make it compatible? 
As far as I understand the metadata of the two FS are different, but what happens to these different metadata?

A:

A file system is actually an abstract user interface to access the data behind it. It works in the same way that you can access data from a DB through a web-page.
You acess this interface with file utilities which create, list, copy, move and delete files, and alter metadata. You'll need then some NTFS utils, ext3 utils and so on  (it's not a given that they will be present).
There are several aspects that the program doing the transfer (for example, nautilus) has to deal with:
-how to deal with long names and non standard characters like blank spaces, non ASCII (normally copying fails here, so better avoid this)
-endianess (the order of storing bytes). It's not the same reading 0A0B0C0D from left to right than from right to left. Both methods are at use, but the problem is old and therefore tools can deal with it, normally.
-things like Linux permissions get compromised when copying files through file systems (when transfering the file, not just accessing them through a file server like Samba). The recipient can change them to whatever he wants, being root and all. File systems like FAT don't support security at all, so as soon as you copy the file to it the security information is simply lost. Linux OSs can apply a standard set of permissions (for example, with umask, not letting any file being executable).

