Ah, it will be hard for me to explain, and I am still a novice when it comes to these kinds of pens. Think of a quill and how people used to employ them when writing. Now, think of a pen with a pointy en. The pointy end is the nib, which ink flows out of. They do not require pressure when writing and should write with the weight of its body. They do not use balls like ball point pens and roller balls, nor do they use any kind of gel ink. I would say they are all refillable, but there are some pens out there that are made to be thrown out, but one thing they all have in common is that they have nibs. Some are more broad than others, an some are extra fine. Three of my pens are extra-fine nibs. I do believe that fountain pens were made years before ballpoint pens, so they have been around for quite a while... maybe after George Washington's time, but I'm not sure. They were made to replace the quill.
Alright, so, fountain pens can improve one's writing, and it helps people with arthritis since they do not need to grip the pen tightly or press hard to make them write like ballpoint pens. They can help make writing look nice. Some pens use pistons, some use convertors, some use cartridges, and some use the barrel (the body of the pen) itself. Ink is stored in the cartridges, pistons, barrel, converters, and are generally refillable unlike some pens that require you to throw away the ink thing in said ballpoint pen, where as with fountain pens, you don't have to unless you're using a cartridge. The converters/pistons do not get thrown away when empty, but instead, you refill them with ink from an ink bottle.
Jinhao 159 As you can see in the poorly taken photo, there is a converter. It can be removed, and the ink would go there. They are meant to be reused, though some fountain pens like Pilot Varsity are to b thrown away once ink has run out. There are different sizes of nibs too!
Here are a couple for you to help you learn more about fountain pens since I feel like I made them more confusing. They are not for everyone, since not everyone likes them. My aunt doesn't, and I think I'm the only one in my family who likes fountain pens.
Fountain Pen Q&A
Fountain Pen 101