Object Snap (OSNAP)

The OSNAP button on the status bar brings up a window that allows you to change the different parts of your drawing that your cursor will snap to.  By default, Endpoint, Center, Intersection, and Extension are turned on.  Midpoint is also very useful to have on for snapping to the midpoints of lines or arcs.  This especially comes in handy if you are mirroring your drawing, and need to know where the exact center of it is.

If you need to snap to a specific part of your drawing that AutoCAD is not snapping to, you can use the Object Snap toolbar.  This tool bar is used rarely and only in complex drawings; the basic OSNAP settings will do for most drawings.