Fountain pen Cartridges.
When you purchase the higher end fountain pen kits (eg. Baron II, Sedona and Roman Harvest Pen kits just to mention a few), you will notice that you get 2 additional parts in the set. The first part is the disposable ink cartridge and the second being a Piston Pump Converter.
1. Disposable ink cartridge - use this if your not a fountain pen expert. Just push it in the back of the fountain pen nib until it clicks. When it is empty, just throw away when empty and buy a new one.
2. For the fountain pen connoisseur. Its called a plunger unit. You use it if you purchase your own ink (eg. http://103.18.196.93/parker-quink-inkbottles.html ) in bottle form. You suck up the ink into the plunger unit. Connoisseurs have their own favorite bottle of ink. The ink comes in different colors and viscosity, you eventually get use to using a particular brand and end up swearing by the product.
To refill a plunger unit, watch this video. The video is a bit dark and poor quality, but it will demonstrate how it is done.