747 autopilot questions

I have some questions about using autopilot on a 747 (and other planes, really).
First of all, can someone explain to me the difference between the various buttons: Heading, Nav, Back Course, Approach (I usually fly using the Heading Hold and Alt Hold but I am curious how Nav is different than Heading Hold, and what Back Course and Approach are).
Second, is there a step by step guide to using the Approach? I know I have to use the ILS freq on Nav1 of the radio but from that point, I'm not sure how the APP works. I have used it once and it seemed to line me up on the runway and I controlled the speed and pitch to come down for a landing. But the next time I used it, the autopilot insisted on flying my airplane towards the ground straight into downtown Dallas (coming in to KDAL). What all does Approach handle for me and how do I use it properly?
First of all, can someone explain to me the difference between the various buttons: Heading, Nav, Back Course, Approach (I usually fly using the Heading Hold and Alt Hold but I am curious how Nav is different than Heading Hold, and what Back Course and Approach are).
Second, is there a step by step guide to using the Approach? I know I have to use the ILS freq on Nav1 of the radio but from that point, I'm not sure how the APP works. I have used it once and it seemed to line me up on the runway and I controlled the speed and pitch to come down for a landing. But the next time I used it, the autopilot insisted on flying my airplane towards the ground straight into downtown Dallas (coming in to KDAL). What all does Approach handle for me and how do I use it properly?