Most walls have cavities – hollow spots created by the wall studs. There are lots of things that run through the spaces, like electrical and plumbing lines. The chances are, everything is protected so that you can nail and drill into most walls and be pretty sure that you won’t hurt a thing.
Even so, whenever you nail or drill into a wall, be careful. And if you encounter unexpected resistance, STOP. Drywall, plaster and wooden studs are pretty soft compared to steel or copper pipes. Don’t push harder on the drill. Instead, back off and investigate the resistance.
As you look at various types at hangar hardware (here are some examples from Home Depot), notice the instructions. They usually spell out the weight and dimension requirements. For lightweight objects there are a couple of choices.
You can use small finishing nails & brads that you drive in at a 45 degree angle into drywall or plaster. Hook type hangers that are held in place with a nail, or adhesive hangers.
To hang medium weight objects you need something a little sturdier. Molly anchors, or hollow wall anchors, are combination screws surrounded by casing. As the screw is tightened the casing around it collapses against the interior wall. Pre-drill a hole and insert nad turn the screw to collapse and tighten.
Toggle bolts are another type of hollow wall anchor. They have spring-loaded wings that expand inside the wall. Pre-drill a hole, remove the wing toggle from the screw and place the screw through whatever you wanna hang. Then replace the toggle and insert the assembly into the wall. Tighten the screw to pull the toggle tight against the inside the wall.
Plastic expansion plugs fit snugly into pre-drilled holes in the wall board. As you twist the screw into the plastic plug, the slatted base at the plug spreads and locks against the perimeter of the hole.
Finally, hanging very large or heavy pictures always requires anchoring the picture hanger into a stud, and you may need two or more hangers for support. You can use a stud finder to find the stud.
Hanging objects on a hollow wall takes a little more time and attention, but doing it right assures that what goes up on the wall will stay on the wall until you take it down.