The term pre-pasted wallpaper means that the wallpaper already has the adhesive applied to its back. This does not mean that it is the backing; just that an additional layer has been added to to the backing to simplify the application process. Typically, pre-pasted wallpaper is used by people who prefer to do it themselves rather than hire someone to do their home improvement projects.
The easiest way to describe the paste is to bring up an item that you are probably familiar with that comes pre-pasted – stamps. While it is a very different kind of paste (the paste on stamps requires very little extra water), it is the same concept. The paste is applied to the back of the wallpaper (on top of the backing layer) and dries before the wallpaper is added to the roll. With the addition of a little moisture, you can hang it on the wall. You do need to apply moisture along the full strip to ensure that the paper holds to the wall.
For the best effect, you can apply additional paste to the pre-paste on the wallpaper to moisten it. This will help ensure the wallpaper is adequately sticky to remain in place as you add it to the wall. If you do add more paste, you need to use something that is very thin so that you do not douse the wallpaper with too much moisture, just like you should not use too much water to moisten it. Over moistening the wallpaper will have the same effect as missing sections of a strip – it will not properly hold on the wall. It will begin to separate from the wall earlier, or it may not properly hold in the first place.
While the pre-paste will make it easier to hang the wallpaper, you will still need to be careful when you hang it. Not only do you have to be careful about the amount of water or paste additive you apply to the back, you have to make sure to go over the paper carefully to keep it from wrinkling or bumping.