"derived from the u-v coordinates"
how exactly is the bitangent direction derived?
The u-v coordinates basically map a vertex (in 3D space) to a 2D texture coordinate, right? The u-coordinate corresponds to the horizontal texture coordinate and the v-coordinate corresponds to the vertical coordinate (in fact, u-v are typically called "texcoords"). So now you can work out the tangent and bitangent vectors by taking the difference in texcoords between neighbouring vertices (interpolated to give per-face TBN matrices, of course).
EDIT: Eric above gives the complete derivation - ninja'ed?