Pine nuts are seeds of pines.
Each seed has a shell that have to be removed before eating. But most pine nuts sold in the shops are already without the shells.
There is about 20 species of pine with seeds large enough to eat.
In different regions seeds are coming from different type of pine, for instance in Europe they are coming from Stone Pine (Pinus pinea). They have been eaten in Europe and Asia for a very long time.
Italian name for pine nusts is “pinoli” / “pignoli” and they are very important ingredient of pesto.
They are used also in chocolates and cookies.
In New Mexico and other areas in US you can find pine nut coffee.