they just define the functions that are contained in some specific Library.
Incorrect. Header files generally declare functions that exist in some library. If they were defined in the header, and you had multiple source files that included that header, you'd end up with multiple definitions of those functions, and it would fail to build.
What can this strange device be?
When I touch it, it gives forth a sound
It's got wires that vibrate and give music
What can this thing be that I found?