I don't know, I think how they're made depends on who you buy them from.
The ones I have bought do not have plastic backing, but they sometimes use a waterproof material called PUL which I heard helps with leaking. But not all cloth pads have PUL in them. Some just use a material called Zorb which is very absorbent (Can hold 10x it's weight in liquids) And I have a very heavy flow, I've yet to leak through my overnight/heavy pads, but I do change them at least once every three hours or less.

Some people say that using cloth pads makes your cramps less painful because there's no chemicals against the skin like there usually are in average disposable pads... It's totally not true. I think they are more comfortable, but they definitely don't make the cramps less painful or less frequent, unfortunately... It'll take a miracle for that to happen lol.

It's actually possible to wash them with your own clothes, however, it's recommended that you don't use fabric softener or those little dryer sheets that you put in the dryer to make clothes smell nice (I hope you know what I'm talking about? I don't know what they're called?)
Using fabric softener and those dryer wipes will compromise the absorbency of the pads, so it's recommended you just use a normal wash without fabric softener.

I prefer sort of pre-washing my pads by rinsing them in cold water and "washing" them by putting them on the floor of my shower, then washing them in the shower with gentle soap. (I'm such a hippie I use organic body wash) Then the blood is usually completely out so it's safe to throw in the wash with the rest of your clothes.