On Win2K with NI-488.2 1.6 and VB5:
ibwrt just takes an instrument descriptor (which you get from ibdev) and a string to send. ilwrt is similar but is just for VB. It requires a byte count, which you can get with the Len function. Send is like ilwrt, but with the added flexibility of specifying a board and GPIB address, plus a termination mode (none, termination character, or EOI signal).
In my opinion, ibwrt is the easiest to use of the three.