Listed below are some common terms used when describing leathers or tanning processes.  There are many, many more terms that get tossed around.

Altered Leather
Leather with its original surface of the skin or hide removed to get rid of imperfections in the original grain surface.  A new grain can be embossed into the leather.

Aniline Finish
Drum-dyed leather finished to have a soft feel, usually coated with protein, resin, lacquer or waxes.

Brush Colored
The application of dyestuff to leather being laid on a table, with a brush.  Some also call this 'hand tipping'.

Buffed
Leather that has been sueded and can also be referred to as snuffed, nubuck or grain-sueded leather.

Calf Hide
Leather taken from an immature bovine.  Calf hides are broken down further based on size of the hide:

Kip:  Calf hide that is less than 14.75 square feet

Extreme:  Calf hide that is between 14.75 sq ft and 18 sq ft

Cordovan
Leather made from the tight, firm shell portion of horse butts that has fine pores and a characteristic finish.  We sell this type of leather here.

Crock
The coloring matter that rubs off of poorly dyed leather.  If it peels off the surface or you find it on your hands after handling, your leather is crocking.

Crust
Leather that has been tanned but not finished.  Crust leathers are often colored/dyed, but no finishing oils or treatments have been added.

Cowhide
OK, this may seem obvious, but cowhide is the hide of a cow.  Rocket science.

Degrained Leather
Leather from which the grain has been removed after tanning, by splitting, abrading or other process.  Very smooth.

Drawn Grain
Shrunken, shriveled or wrinkled grain surface of leather.

Distressed
Buffing surface to create uneven coloration and markings for a weathered look

Embossed
Prints, commonly reptile or western patterns, are heat and/or pressure pressed onto leather surface.  Some will use this term interchangeably with words like 'plated' or 'fake grain.'

Fat Wrinkle
Wrinkles in the grain of leather caused by fat deposits in the animal.  Yet another reason to get your cows into a gym.

Finish
A surface application on the leather to color, protect, or mask imperfections.  Glossy clear coats, for example.

Full Grain
The outside original skin or hide that has had the hair removed, but has not been corrected or altered. It possesses the original grain of the animal.

Grain
The outside of the hide or skin consisting of the pores, wrinkles and other characteristics that constitutes the natural texture of the leather.

Grain, Embossed
An artificial grain pressed into the surface of top grain leather with original grain removed.  Like plated leathers.

Glazed Finish
Surface is polished to a high luster by pressurized glass or steel rollers, to yield something like patent leathers.

Leather
A generic term used for all kinds of tanned animal hides or skins.

Matte Finish
A flat or dull finish.

Mineral Tanned
Leather tanned by mineral substances like the salts of chromium/ aluminum/ zirconium.

Napa
Commonly refers to the surface or top grain of any soft leather hide.

Natural Grain
A leather that retains the full original grain.

Nubuck
Lightly buffed top grain to a very fine nap that appears smoother than suede.

Oil Tanned
Leather tanned with fish oils giving a soft and pliable leather like chamois.  Luckily, it doesn't smell fishy at all.

Patent
Heavily finished to give a highly lustrous, shiny appearance.

Pearlized
Spray-on finish giving pearlized (shimmery) effect.

Pigskin
From pigs or hogs, commonly used for suede.

Perforated
The process of punching, piercing, or die cutting small holes to form a pattern in the leather.

Plating, Plated Leather
Pressing leather with a heated metal plate under high pressure.  Similar to embossing.

Reconstituted Leather
Material composed of collagen fibers obtained from ground up hide pieces that have been constructed into a fibrous mat.

Saddle Leather
Vegetable-tanned cattlehide leather for harnesses and saddles, usually of a natural tan shade and rather flexible.

Shearling
Natural lamb/sheep pelts with the leather side often dyed and the hair typically cropped (sheared) to some uniform length.

Side (or Side Leather)
Half of a full cowhide, cut right up the backbone.  In general, sides run about 18 to 22 square feet of total surface area.

Split
When a thick hide is split, the term refers to the top surface which looks like suede but is not as soft.  Splits are left overs, but can be made into excellent leathers.

Suede
Leathers that are finished by buffing the underside of a hide to produce a velvet-like nap.

Snuffed
The grain surface abraded with brushes, emery wheel or sandpaper. It is done to remove defective grain or for sueding the surface of leather.

Top Grain
The outside surface of the animal, often buffed or sanded to smooth the top of the leather and make it uniform and smooth.  Some top grain leathers are embossed with imitation patterns (lizard, gator, etc).

Trim or Trimmings
The removal of parts of a skin or hide not suitable for making leather, such as portions on the outer edges.

Unfinished Leather
An aniline-dyed, naked leather with no additional application intended to finish, color or treat in a way that would alter the natural characteristics of the leather.

Weight
The weight of leather is measured in ounces per square foot.  More information on thickness is available here.