A nib is a piece of metal (steel, gold plated steel, solid gold (of various karats) are typically used) shaped to a point, with a tiny ball of superhard material affixed to the tip (usually iridium or ruthenium). It is then sliced from the tip to the mid-section of the nib to allow ink to flow into the crevice. The nib is then put on the feed
Nib size determines the the thickness of the line that the nib draws. The most popular sizes, and therefore the most prolific are Extra Fine, Fine, Medium, and Bold. With Fine and Medium being the easiest to find, and Extra Fine and Bold the rarer of the&group. You can get almost any pen in one of these sizes, though you may have to special order it if the store doesn't carry the size you'd like. An odd note are oblique nib
Next, there is the issue of smoothness of writing. This has to do with how well the super hard tipping material has been polished. You will find that solid gold nibs are smoother typically than are steel or plated nibs. SGs are typically smoother than steel because, the maker is willing to polish a higher priced nib
longer. This is by no means a rule. I have some steels that compete with SGs for smoothness.