A tailor is someone who can make clothing from cloth (& some leather). It is ideal for classes who are restricted to wearing clothing categorised as 'cloth' (Mage, Priest and Warlock). They can make special outfits also. Their most popular use is as bag makers, either to supply friends or guildmates and also to sell on. There is no obvious supporting profession for Tailoring but higher level items will require reagents created with other professions such as Frost Oil & Fire Oil (Alchemy), Enchanted Leather (Enchanting) and Iron Buckle (Blacksmithing) to name but three. You can always trade for the few components that are exclusively gained from another profession.
Alchemy
Blacksmithing
Mining
Herbalism
Leatherworking
Skinning
Engineering
Tailoring
Enchanting
Secondary Proffesions
Cooking
First Aid
Fishing