Pilum heavy (Photo credit: Wikipedia)
English: Cambodunum Pilum (Photo credit: Wikipedia)
Nearly all Roman soldiers carried throwing spears known as Pilum, or Javelin. These were used mainly by skirmishers who would have three or four Pila (plural for pilum). These skirmishers (or Velites) would go forward in front of the main heavy infantry in very loose formation and throw their pila at the enemy lines. They had no amour, so the enemy infantry were unable to catch up with them and stop them. However, what the Velites were really good against were elephants. Arrows normally could not pierce the tough hide of war elephants, but javelins could. Also, the Velites were normally spread out, so less got killed in an elephant charge.
The Pila themselves were made of a long wooden handle or shaft, which was attached to a long thin iron shaft which ended in a triangular head. The iron was made very soft, however (it seems like the Romans figured out annealing), and the reason is because when a Velite threw his Pilum, it would (if he is lucky) either stab his target to death, or it would puncture the enemy’s shield. Because the iron was so soft, it would bend upon impacting the shield, but puncture as well. This meant that that soldier now had a javelin stuck in his shield. If he tried pulling it out, he would have to stop and work on it for a bit. This he could not do while in formation, so he would just have to leave it in his shield or drop the shield, as the Pilum was heavy (some sources say they had lead balls attached to increase throwing power) and cumbersome, and really decreased movement.