Leatherworking as a profession is in some ways quite similar to Tailoring. Just like tailoring, leatherworking allows one to craft different kinds of equipment/gear to boost certain stats of a character. While tailoring mainly focuses on different types of cloth that is dropped 
from monsters in World of Warcraft, leatherworking depends heavily on leather which can be scavenged/skinned from beasts in the world.
Besides being able to craft different kinds of gear for yourselves and others (to make some side cash), leatherworking does have some additional goodies. To name a few, leg armor patches, different kinds of drums that boost certain type of stats for a stipulated amount of time, quivers and ammo pouches that can be used by hunters, Mining and Inscription bags used by Miners and Jewelcrafters to store ores and jewels etc.. etc..
And last but not the least are the leatherworker-only patches, also known as Fur Lining which was introduced in Wrath of the Lich King patch. These patches are incredibly useful as they boost a certain types of stats of your choice, which can be applied to your bracers. Of course this would negate the ability to have a normal enchant on that particular slot but these leatherworking-only enchants/patches are far more superior to normal enchants that Enchanters can normally do. This is quite similar to Inscription specific shoulder enchants or Jewelcrafting specific gems or Tailoring specific cloak - embroidery or Engineering specific hand mounted pyro rockets.
Typically most leatherworkers go with skinning as their secondary profession. They both go very well together, as most of the leather you would end up skinning while leveling would get used to skill up on leatherworking. Although it's not necessarily needed as a secondary profession it is indeed recommended to pick skinning as your secondary profession if you plan to choose leatherworking as your main.
Another peculiar characteristic of this profession that is similar to some other crafting professions in WoW is the ability to choose specialization. At 225 skill level you will get to choose between 3 specializations, namely - Elemental Leatherworking, Tribal Leatherworking and Dragonscale Leatherworking. Elemental helps one make gear best suitable for feral druids or rogues. Tribal is best suited for Moonkin druids. Dragonscale produces mail armor for hunters and caster shamans. However after Wrath of the Lich King was introduced, this specialization doesn't really benefit that much since most of the epic gear can be crafted without having to choose any one particular specialization. So it is merely a personal choice with respect to your class and the talent points you choose.
Approximate Materials to be gathered:
Ruined Leather Scraps - 57
Coarse Thread - 55
Light Hide - 40
Medium Hide - 10
Salt - 140
Light Leather - 170
Medium Leather - 150
Fine Thread - 140
Gray Dye - 15
Heavy Hide - 30
Heavy Leather - 305
Bolts Of Silk Cloth - 20
Thick Leather - 385
Silken Thread - 150
Rugged Leather - 455
Black Dye - 40
Rune Thread - 40
Knothide Leather Scraps - 100
Knothide Leather - 540
Fel Scales - 28
Borean Leather - 1830
Crystallized Water - 75
Arctic Fur - 10
Nerubian Chitin or Icy Dragonscale - 10
Frozen Orb - 5