Macfuse is a collection of headers. 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. Note that it does not load any drivers or affect your system in anyway!
MacFUSE is not an application. It's not a background process. It's not something that launches automatically at boot time or any other time. It does not configure, reconfigure, or modify the normal working of OS X by itself. It does not alter settings of any OS X components besides placing its preference pane for System Preferences to use.
It is essential for other utilities such as ntfs-3g, macfusion, to work.
