ALRT-As far as I can tell, this resource mainly determines where a certain alert dialog box will appear on the screen. ResEdit displays the whole screen in miniature and you may then reposition the dialog window by just dragging it. This may do more things (probably does); I dunno.
CURS -Cursors, like the familiar arrow or the watch. ResEdit lets you edit these Fatbits-style.
DITL-This stands for Dialog Item List, and contains all the elements that comprise a given dialog or alert box, including STR#'s, ICON's, PICTs, etc. Combined with the DLOG resource (see next line below), these make up the message boxes the Mac throws at you on occasion.
DLOG-Similar to the ALRT resource (see above) in display and usage.