A nib is the part of a quill, dip pen, or fountain pen that deposits the ink to the writing surface.  Whether feather, or metal, they are shaped in a way that takes advantage of capillary action, surface tension, and gravity to encourage ink to flow toward the writing surface.

From Wikipedia Common

          There are many types of nibs.  They vary according to their purpose, shape, size, and the material of which they are made.

Quill
          Quills are the large primary feathers taken from a bird's wings.  These feathers are hollow (calamus) near the writing end; where the nib will be shaped.  The calamus serves as a reservoir for ink, and, when the tip is treated and shaped, the feather will make a beautiful writing instrument.
          The traditional tool for shaping a nib on a quill feather would be a 'pen knife'.

From Wikipedia Common

Dip Pen
          A nib pen is a nice way to have a custom handle made for you, or by you.  As you can see in the image above, they can be made of any material you choose as long as they will accept a pen nib at the writing end.  They can be a nice Fimo project for kids.  You don't have to use a high priced Fimo, any polymer clay will do, but it has been my experience that Fimo brand has brighter colors after baking.  My personal favorite is a nib pen turned and shaped in wood.

From Wikipedia Common

Fountain Pen
          There are many nibs available for the modern fountain pen.  Unlike the nibs shown above, many have a round bead of metal at their tip to soften their lacerating effects on the paper.  The early nibs made of gold had a small ruby ball attached to their tips.  Gold has been replaced with high quality stainless steel, and the ruby has been replaced with iridium or platinum.

"IRIDIUM POINT GERMANY" From Wikipedia Common