It has decent defensive capabilities but is not an especially deadly weapon, though it is nearly as effective as many other spears and can be thrown a short distance. The trident is a two-handed weapon and its combat effectiveness is modified by the Spear skill.
URW Game Encyclopedia description
The official URW in-game encyclopedia (accessed by the key) describes the trident thusly:
Trident is a triple-pointed spear. Trident is commonly used in fishing. When used as a combat weapon it attacks no better (sometimes worse) than a spear, but its three prongs gives it a somewhat better defensive capacity.