What you might consider CutAndPaste in XFree86 (select and middle click) is NOT; it's DragAndDrop. Go read about it.
freedesktop.org, the people trying to make GNOME and KDE play nice, don't have an official specification about cut and paste, but apps tend to use the X Clipboard selection buffer. JamieZawinski explains it in his X Selections, X Cut and Paste article.
Ctrl-C is the standard 'copy to clipboard' key in MicrosoftWindows; since Unix uses it for Break you'll quite often see Ctrl-Insert or Shift-Ctrl-C used as the 'copy' key in terminal emulators.