If you're not a seasoned pearl buyer, you may get a little lost confronted with so many different terms describing types, shapes and colour.  I thought I'd give a little help with a quick guide. Lots more in-depth information can be found on dedicated pearl sources, if you're so inclided.  Let's start with the two main differentiators:

CULTURED V NATURAL

A pearl is formed when a foreign body finds itself inside the oyster; the mollusc starts building nacre (mother of pearl) on it, eventually creating a pearl. Natually formed pearls, by accident, are very rare. The vast majority of what's used in jewellery these days is cultured pearls, where the oysters are injected with tiny pieces of nacre, to encourage the building of pearls.  Cultured pearls are therefore controlled in terms of shape and size production.

FRESHWATER V SALTWATER

Freshwater pearls are cultured pearls from molluscs grown in lakes and ponds. China is the main commercial producer of these pearls today. Freshwater pearls come in a great variety of hues and sizes. They also have much higher yield than saltwater pearls, due to the fact that the molluscs are injected with multiple seeds at a time. This makes them more affordable and therefore quite popular as a choice for jewellery design.  

Saltwater cultured pearls vary according to provenance - Akoya, Tahitian and South Sea.  Here is a quick comparison:

SHAPE

Pearls come in a variety of shapes, from the symmetrical to the entirely freeform.  Here are the most common shapes, though many more can be found (coin-shaped, shards, etc.):

And finally... being natural (although cultured), individual pearls are unique.  The greater challenge in jewellery is to source matching pearls: even two white pearls won't necessarily have the same white hue, two round ones won't be exactly round, the sheen won't be the same.  Imagine the challenge in a single pearl necklace!