Xt: The X Toolkit Intrinsics is a library layered on Xlib which provides the
functionality from which the widget sets are built. An "Xt-based" program is
an application which uses one of those widget sets and which uses Intrinsics
mechanisms to manipulate the widgets.

Xmu: The Xmu library is a collection of Miscellaneous Utility functions
useful in building various applications and widgets.

Xaw: The Athena Widget Set is the Consortium-implemented sample widget set
distributed with X11 source.

Xm: The OSF/Motif widget set from the Open Software Foundation; binary kits
are available from many hardware vendors.

Xhp (Xw): The Hewlett-Packard Widget Set was originally based on R2++, but
several sets of patches exist which bring it up to R3, as it is distributed
on the X11R4 tapes. Supplemental patches are available to use it with R4 and
later.

CLX: The Common Lisp X Interface is a Common Lisp equivalent to Xlib.

XDMCP: The X Display Manager Protocol provides a uniform mechanism for a
display such as an X terminal to request login service from a remote host.

XLFD: The X Logical Font Description Conventions describes a standard logical
font description and conventions to be used by clients so that they can query
and access those resources.

RTFM: Common expert-speak meaning "please locate and consult the relevant
documentation -- Read the Forgotten Manual".

UTSL: A common expression meaning "take advantage of the fact that you aren't
limited by a binary license -- Use The Source, Luke".

API: Application-Programmer Interface. The function calls, etc., in a
programming library.

BDF: Bitmap Distribution Format; a human-readable format for uncompiled X
fonts.

GUI: graphical user interface.  

UIL: the User Interface Language, part of OSF/Motif which lets programmers
specify a widget hierarchy in a simple text "outline" form

WCL: the Widget Creation Language, a package which extends the understanding
of the Xt resource format such that a widget hierarchy and actions on the
widgets can be specified through the resources file

UIMS: User Interface Management System