Gen field
Hi,
In a generator, a spool of wiring is rotated inside a magnetic field. Inside the wiring elctron movement is induced, in other words, electricity.
The magnetic field can be the one from a permanent magnet, the Gen is then a Permanent Magnet Generator (PMG), or the magnet can be an electro-magnet, usually a weak-iron piece with electric wire coils wrapped around it. The current that is passed through the coils of the electro-magnet is the so-called "field" that can be switched off to stop the rotating spool from generating electricity.
Hope this explanation helps you.
Cannot help you on the horn question.