XFS is the abbreviation of X Font Server. An X Font Server, as the name implies, is a server that produces fonts. For example, when making a remote connection to an X server, the X Font Server will serve fonts to the client. 

Fonts typically reside in one of the following directories:

/usr/X11R6/lib/S11/fonts
/usr/share/fonts
/usr/share/X11/fonts

In the fonts directory will be files and folders. Files with a .pfa or .pfb extension are Type1 fonts. Files with a .ttf extension are True Type fonts.

Let's say you obtain a fancy font named pacifico. You will want to move the pacifico file to the appropriate font directory. It is a best practice to create a unique directory for font you download. For example, a directory named local can be created at /usr/share/fonts/local. Let's say the pacifico file is moved to the /usr/share/fonts/local/ directory.

The fonts configuration file is located at /etc/fonts/fonts.conf. This file should not be edited by hand.