What is BITNET?

[JDS5.TYM%office-2 at sri-unix.UUCP]

Would someone out there please send me some information about BITNET?  I don't 
want technical info, just a brief description of the

purpose of BITNET , what it is, who supports and uses it and that sort of stuff.
 A friend of mine asked me about BITNET and I think I remember seeing some mail 
with that name in the headers.

Thanks very much,

Jeffrey Stone,  Menlo Park, CA