Anywhere they'll stick. Well, anywhere they'll stick that is legal and anywhere they'll stick.
This is a pretty open-ended question, so I'm going to give you a quite detailed answer on places that vinyl stickers and labels (a.k.a. decals) can and are used.
Automobiles. From bumper stickers to window decals, you can decorate your vehicle with car window stickers that advertise your school affiliation, what university you graduated from, what political party or candidate you support, what charity you are passionate about, or what products you endorse.
Signs. Many signs use some form of vinyl sticker as letters and logos, either as exterior or interior mounted signs or as signs applied to windows or larger vehicles like delivery trucks and semi-trucks. You may have seen entire sides of semi-truck trailers advertising your favorite fast food restaurant cruising down the interstate highway. That graphic is a large format vinyl sticker or decal.
Product Labels. While many product labels are made from paper products, significant quantities are also made of vinyl decal material. Wine bottles, shampoo bottles, food labels. The sky is the limit. If you're interested to discover more about graphic labels, view here.
Warning Signs. Many warning signs that you see here or there are vinyl stickers or decals. These stickers are used on pipes, doors, floors (marking fire escape routes, for instance), barrels of hazardous materials, and lots of other places that are often required to post warning signs.
Types of Wall Surfaces for Decals
That's a multi-faceted question, so I'll need to give you a multi-faceted answer.
Generically, your wall surface needs to be clean, and preferably freshly painted, but not too freshly painted or you may have problems with your wall graphics not sticking (because the paint is too damp), or sticking too well (surface may be dry but there is still residual moisture below the surface).
The wall should not have too much texture. An orange peel surface should be OK in most instances, but rougher surfaces may not hole your graphics on the all if there's too little contact with a wall.
Wood, Metal, and Board
Commercially speaking, you should be fine with painted wood, metal, or gypsum board (sheet-rock). In my experience, oil-based paints are a better surface to apply vinyl wall murals to than latex, but oil-based paints are more expensive and harder to find in the 21st century, so it is likely that you may not have a choice but to apply your wall vinyl graphics to latex paints. Just try to give them at least a week, but preferably two weeks, to cure.
Glass Windows or Walls
Finally, you can apply wall graphics to glass if you choose to with no negative effects. The only caveat I have is that it is much easier to trap air bubbles underneath a vinyl graphic if you don't know what you're doing or have limited experience applying vinyl letters or graphics to glass. It might be advisable to hire a professional in this instance.
ABOUT THE AUTHOR
Barry K. Brown has been in the Sign, Banner, Decal and Display Business for over 20 years. It isn't what he thought he'd do with his life, but he says he knows too much now to do anything else!
He has been marketing these products online since 1998, and the company he was general manager of in 1998 was the first sign company to be listed on Yahoo!