difference between power wire and ground?

is there a difference between power wire and ground wire or can i use power wire as ground wire?I am looking in to hooking up two batteries in parrelel. Somebody told me to run 0 guage wire from the positive to the positive, and the negative to the negative. Somebody else told me told me just to run 0 guage from positive to positive and then ground it to the chassis. Which would work so that it charges the secondary battery?

when your hooking up 2 subs in series-parrlel , which speaker wire do you use to go from the positive to the negative? Do you use the negative wire or the positive wire or both?

What about hooking up a single sub in series? Which wire do you use?