In leatherworking you primarily create leather armor - and mail armor later on. You can also create ammo pouches and quivers and turn one kind of leather into more advanced leathers. Leatherworkers can also make armor kits and in The Burning Crusade there are definitely some popular ones with great stats (for example  Nethercobra Leg Armor). Another new type of item leatherworkers can make (but only use themselves) is drums - they can be used in battle for great advantages. These have great utility to boost a party/raid and usually cost little to make - which is good for skillups. The only downside is that it usually takes a lot of faction grinding to get these recipes.

Leatherworking is most useful for hunters, shamans, rogues and druids.

Like Blacksmithing, leatherworking is very dependant on a gathering trade for resources - Skinning. If you don't have a high level Skinning alt then Skinning is a must unless you want to part with most of your beloved gold.

If you do have Skinning on a high level alt, then other professions to pair up leatherworking with could be Enchanting, Engineering, Blacksmithing, Jewelcrafting or Alchemy. You could also choose another gathering trade that you don't already have on an alt to make more gold. Take a look at the other guides to find out which trade is for you.