Got this news from here :

FUSE stands for Filesystem in USErland, and it provides a generic interface that lets the operating system see virtually anything as a filesystem. Historically, adding new filesystem recognition to an operating system has meant modifying the kernel for each new FS. FUSE, though, provides a single interface that filesystem modules use to interface with the OS. Best of all, anything that provides the correct interface can be interpreted as a filesystem. One enterprising Python programmer even developed a script to let users mount their GMail accounts and use the extra space in their accounts to save files.