Rather than reinvent the wheel, let me direct you to the NCSA Imagemap Tutorial. Ignore the first part about compiling c programs, etc., page down until you get to 'Your First Imag e Map'.
Now that you're a factory-trained expert in imagemap theory, let me give you a specific IONet example:
Let's say I have an image in the ~mrbill directory (my personal www directory on the IONet server) called 'iologo.gif' that I want to use as an imagemap. First, I would use a program such as ma pedit to create my .map file. I save the file as 'iologo.map', then transfer it to my personal www directory on the IONet server. When writing my HTML page, I would call up the map in the following manner:
<a href="http://web.archive.org/web/199603/http://www.ionet.net/cgi-bin/imagemap/~dbynum/iologo.map"> <img border=0 src="[unarchived-media]" ismap>I put the 'border=0' command in my IMG SRC tag to remove the usual blue or purple border from the image. You may or may not wish to do this.
That's about all there is to it. (And you thought this was going to be hard....)
I am looking forward to seeing many creative and, umm, esoteric maps out there on the user pages. Go thou forth and create!