The brachial plexus is a network of nerves formed by the anterior rami of the lower four cervical nerves and first thoracic nerve (C5, C6, C7, C8, and T1).
These six divisions regroup to become the three cords or large fiber bundles. The cords are named by their position with respect to the axillary artery.

The posterior cord is formed from the three posterior divisions of the trunks (C5C8, T1)
The lateral cord is formed from the anterior divisions of the upper and middle trunks (C5C7)
The medial cord is simply a continuation of the anterior division of the lower trunk (C8, T1)