Introduction
Materials which conduct electricity are metals (iron, copper, alluminium et cetera) and also carbon, in the form of graphite. All other types of material are electric insulators.
However, these insulating materials (like plastic) can be charged by friction. This is because the material is either loosing or gaining electrons.
Electrostatic phenomena
This charging of insulative materials can be seen in the everyday world. For example, if you have a jumper made of synthetic fabric (polyester), when you take it off you may find your hair standing on end because it has rubbed against the jumper. You can also get shocked by the rubber handrail of an escalator. This is because it has gradually become charged over the course of its rotation, and electrons are being transfered.
This electrostatic charge can have some serious dangers (other than ruining your hair). As an aircraft is being refueled the friction causes charge separation: where the aircraft has an opposite charge to the fuel. If the voltage becomes high enough it may cause a spark. Igniting the fuel and causing a fire.
To prevent the framework from building up charge, it is connected to earth, so that any charge can disperse, and no fire.
As well as dangers, electrostatic charge has many uses. A common one is in painting metals ... how does your car have such a smooth flawless paint work?
As the paint comes out of the nozzle it is positively charged, causing it to separate because like charges repel. The metal plate is earthed, and electrons flow from earth causing it to become ngatively charged. The paint is attracted to the metal because opposite charges attract so there is ab even coverage.