Yes - the existing file will be replaced.
If more than one file is being copied and this would cause any other
files to be replaced, this dialog box will reappear for that file.
No - the existing file is not replaced
and the copy is cancelled for that file. If more than one file
was being copied, the other files will still be copied. If this
would cause another file to be replaced, then this dialog box will
reappear for that file.
All - the existing file is replaced
as are any other files if more than one file is being copied.
This dialog will not reappear and all target files with the same name
as a source file will be replaced. Beware!
Cancel - the existing file is not replaced
and no further copying of other files is done. Some file may
have already been copied though by this time.