If you join two work pieces with screws, you can - in contrast to glue or nails - separate the work pieces again without damaging them.

In just the same way as you use a suitable drill bit for drilling in each material, you need the right screw for screw driving into each material. For wood or chipboard, you use different screws than for thin metal plate - but you can insert both directly into the material without needing a hole that has been pre-threaded. Machine screws, on the other hand, are used with matching nuts or screwed into threads.

If you want to secure something to the wall, you need not only the screw but also a 'wall plug' type fixing. This ensures a secure hold. The wall plug is pressed into the hole after drilling. The relationship between drill bit and wall plug size: wall plug diameter corresponds to the drill bit diameter. The exception to this rule is very soft building materials. This formula applies
wall plug diameter – 1 mm = drill bit diameter.
This means that the wall plug holds better. Note: The correct drill bit diameter is nearly always indicated on the packaging of the wall plugs.

The classic type of wall plug is the general purpose one made of plastic, which expands in solid masonry as the screw is driven in. It expands in solid masonry; in hollow masonry or behind board materials such as plasterboard or wooden material boards, it forms a knot when the screw is inserted. 

For securing in cavities, other fixings, such as rocker or folding spring dowels made of plastic or ‘cavity fixings’ made of metal are suitable. These have an enlarged surface area and are better able to distribute the forces acting on them. If you want to secure heavy loads to the ceiling or wall, you should really use these specialists.

Naturally, the screw and fixing must match up. The diameter and length of screws needed for each fixing type is shown on the fixing packaging.