how is DecWindows related to Xwindows? compiling Xprograms...

[jay dubb]

   I am a C programmer, but know nothing whatsoever about windows,
Xwindows, etc. What I am wondering is, I have an Ultrix 4.1
workstation with the DecWindows woprkstation software. Does this mean
I can compile all the nifty public domain Xprograms available by anonymous
FTP, or is X-windows a completely different sort of beast? I noticed
that I do have a directory /usr/include/X11, but it is empty. Can
anyone enlighten me about this whole windows business?