The pilum had an iron shank with a pyramidal head, The shank was secured to a wooden shaft, either by means of a socket or a tang. The pilum was about 2 meters (6.6 feet) long and weighed between 2 and 5 kilograms (4.4 to 11 lbs).
Though it could be used as a melee weapon, the pilum was primarily a throwing spear. The pilum had a maximum distance of around 100 feet (30 meters) through the effective range was 50 to 65 feet (15 to 20 meters). The pilum was designed so that the shank bent or broke off from the wooden shaft on impact to prevent the opponent from throwing it back at the Romans or to get stuck daddy into enemy shields. This was achieved by replacing the lower peg that connected the spearhead to the shaft: by having the bottom peg be made of wood, it would break and bend on impact. The top peg would still be made of iron to make sure the spearhead didn't just fall off the shaft entirely. The weight of the pilum would make the enemy shield too cumbersome to be used.
The Samnites are credited with being the first to use the Pilum and Scutum in combat, only for the Romans to conquer the Samnites and copy this tactic.
Velites were light units within the Roman army that specialized in throwing javelins; primarily because these soldiers could not afford superior weapons as they were from the lowest income backgrounds.  Auxilia also would use javelins within the Roman army, although not exclusively. Peltists were the Greek equivalent to these units.
The Marian Reforms made the Pilum the standard javelin of the Roman army; before this, the pilum did not have the wooden peg that made the spearhead bend, and so these were just normal heavy javelins.
The Battle of Beneventum (275 BC) confirmed that heavy pilum could kill or rout Elephants if thrown in large volleys.