Tailors take simple cloth and weave wonders out of it; cloth armor and robes, shirts, bags and other creations are the purview of the tailor. The benefits are diverse - lightly armored classes can wear mystic robes, all types of heroes can benefit from a variety of bags, and everyone can appreciate the decorative shirts and outfits springing from the tailor's loom.
Tailoring doesn't rely on a gathering profession - cloth is found in chests and on defeated foes, as well as purchased. As such, Tailoring is paired with a variety of professions, though Enchanting is common.
Most Tailoring can be done anywhere, provided that you have the necessary materials.
To tailor items, first ensure that you have learned the pattern you'd like to create, and that you have the required cloth, dyes and other special materials listed for that pattern. Cloth can be acquired from many enemies (or often from the auction house), and other materials are available from trade goods vendors.
Once you have the materials you need, you can tailor at any time and nearly any location - simply choose 'tailoring' from your professions menu, choose the item you'd like to make, and click 'create'.