MacFUSE is software that allows you to write arbitrary file systems as user-space programs. You can think of it as a library for easily developing Mac OS X file systems. Another crude way to look at this would be to think of MacFUSE as something that makes Mac OS X work like a microkernel for the purpose of writing/running file systems. MacFUSE has two major components: an in-kernel loadable file system and a user-space library (libfuse). The in-kernel file system is specific to Mac OS X and is not based on Linux FUSE. (Some of its code is based on the FreeBSD implementation of FUSE.) The user-space library (libfuse), which provides the developer-visible FUSE API, has numerous Mac OS X specific extensions and features.