help, need wheel spacers for dummies

[280z/300zx]

So while not new to the world of cars or mods I know almost nothing about spacers. I've heard it improves cornering by giving wider wheel base. I've heard it can also improve launching a car at the track and decrease wheel hop. I've also heard it can decrease relaibility of the hub. Basically I'm looking for someone to break down wheel spacers for me. I want the good, the bad, and potential problems that can occur. Also how to decide what mm you go with. also can you do just the rear or do you have to do both front and rear. Thanks and sorry for the newb questions.