A filesystem is the way the files are stored in a hard disk.
Let's imagine a clean hard disk as an empty room that you are going to use to store things.
There are thousands of ways to organize those things in the room to make it easy to find any of those things when you need it.
FAT, EXT4, NTFS, etc. are simply ways to organize the files in the hard disk. The main points are:
- Quick find any file
- Easy deletion of files.
- Recovery of errors in read or write proccess.
Apple OS X uses HFS+ and Microsoft uses NTFS.
They are simply different ways to put the same information on the disk.
The contents of the file are NEVER modified.
As NTFS and HFS+ are propietary filesystems, each vendor is not interested in compatibility with the other one.
FAT32 has become the de facto filesystem for portable systems (USB drives for example), so Linux and Apple have been forced to include drivers for FAT32 to be compatible with the USB devices in the market.
However Microsoft is the owner of a series of patents for key parts of the FAT file system.
Microsoft offers licenses for use of its FAT specification and "associated intellectual property", at the cost of a US$0.25 royalty per unit sold.
When you send an attachment in an email, you are simply reading from the filesystem and putting the file (remember, that the content never is modified) inside the message, and when other one receive the email with the attachment, he is simply putting it in his hard disk.