The Converter is a small filling mechanism. It fits on a pen that can also accept a cartridge.
Nib: the metal tip of a fountain pen that places the ink on the paper.
Cap: the part of a pen that covers the nib and connects to the pen body.
Barrel: the long section of the body of the pen which holds the ink. I can recommend the following pen parts kit. https://www.amazon.com/Gentleman-Fountain-Pen-Kit-3-Pack/dp/B074F9ZTYT
A fountain pen works by both gravity and capillary action. Traditionally they were made using quills. A small hole drilled into the end of the quill allowed the ink to flow. The feed of a fountain pen is the component that connects the nib of the pen with its ink reservoir. It regulates the amount of air that flows backwards up to the reservoir and then replaces the used ink. It’s what give you what is referred to a Controlled Leak based on an even flow to the nib.