what neuro said, plus.....
-zone AF (I like that sometimes - with one click you move AF a whole quadrant)
-you can register an AF point
-you can assign an AF point to the MFn2 button
here's what I do..
I have 3 AF points available - an upper left exp point assigned to the back button AF point, an upper right AF point registered to the metering start button (star next to the AF-ON button), and the center AF button assigned to MFn2
at any given time I have single-button access to focus on left-, right-, or center-AF points
also, customized for horiz and vert orientations