plugs are made from metal(usually brass) and plastic
Metal to conduct (electricity)
Plastic (case) to protect from electric shock
It works by connecting live, nuetral and earth cables in an electrical product to the live, neutral and earth feed usually located in the socket in the wall.
Im no electrician but I hope this helps in some way. Strange question why the need to know.
Unless you meant a sink plug. but then again they are usually metal and plastic but try not to get the sink anywhere near electricity especially if there is water in it