What does the argument list in C-h f delete-duplicates mean?

Hello, XEmacs.

When I do C-h f delete-duplicates, the resulting text starts off:

`delete-duplicates' is a built-in function
  -- loaded from "/home/acm/emacs/xemacs.hg/xemacs/src/sequence.c"
(delete-duplicates SEQUENCE &key (TEST #'eql) (KEY #'identity) (START 0) END FROM-END TEST-NOT)

.  What does this mean?  In particular, I have never seen parenthesised
expressions, such as "(TEST #'eql)" inside an argument list before.  The
XEmacs manual (page "Name Help") doesn't explain this.  Are TEST, KEY
and START some sort of optional parameters, or what?

Thanks for the elucidation!

-- 
Alan Mackenzie (Nuremberg, Germany).