I found this that explains it, or some of it anyway -
[ * The five roots are the five anterior rami of the spinal nerves, after they have given off their segmental supply to the muscles of the neck.

* These roots merge to form three trunks:
o "superior" or "upper" (C5-C6)
o "middle" (C7)
o "inferior" or "lower" (C8-T1)

* Each trunk then splits in two, to form six divisions:
o anterior division of the upper, middle and lower trunks
o posterior division of the upper, middle, and lower trunks

* These six divisions will regroup to become the three cords. The cords are named by their position in respect to the axillary artery.
o The posterior cord is formed from the three posterior divisions of the trunks (C5-T1)
o The lateral cord is the anterior divisions from the upper and middle trunks (C5-C7)
o The medial cord is simply a continuation of the anterior division of the lower trunk (C8-T1)
]
there's more here with images and diagrams on the page-
http://en.wikipedia.org/wiki/Brachial_plexus