As I recall, back in the old days, ISO refered to the sensitivity of the film to light. You adjust your aperture/exposure combination to suite the film. Does anyone have the lowdown on how a digital camera does this?
Aperture priority is where you choose the aperture and the camera choses the exposure that works best. An old camera I have has Exposure Priority where you set the exposure and the camera choses the aperture. I haven't seen that in a while but I haven't looked very hard.
Depth of field is how much of the picture is in focus. A small depth of field will have the subject in focus and the forground and background fuzzy. A large depth of field will have the subject, the foreground and the background in focus.
The depth of field is determined by your aperture (f-stop). A large aperture lets lots of light in but narrows the depth of field. A small aperture needs a longer exposure but gets more things in focus. If you can still afford film, go play with a pinhole camera sometime (they have a very small aperture, long, long exposure but EVERYTHING will be in focus).
With new cameras you can choose the ISO to suite the aperture/exposure/depth of field you want. Back in the day, we had to change the film to change the ISO.