A longbow is a natural composite bow with a length of 4 to 6 feet.
It is not craftable and needs to be bought by the player character via trading.
URW Game Encyclopedia description
The official URW in-game encyclopedia (accessed by the key) describes the longbow thusly:
Any bow between four and six feet high, generally the same height as the archer. The bow is carefully cut from the heartwood and bodywood of a branch, and is in effect, a natural composite bow. The tips are usually sheathed with horn and the string is generally made from hemp. The longbow has amazing power and accuracy, but it takes years of training to become proficient in its use.