A Javelin is a sort of light spear, to be thrown or cast by the hand.
Anciently, it was a weapon of war used by horsemen and foot soldiers.
The Romans specially designed the head of their "pilum" so that it bent on contact with the ground or the target. This prevented it being thrown back at the soldier. If it was lodged in a shield etc., then it was hard to extract.
In the Middle Ages, javelins were used chiefly in hunting the wild boar and other fierce game.
In the SCA, javelins are a missile weapon, and being such must have a pressure point. Because they are thrown, not propelled by a bow or siege weapon, they may be used by heavy fighters.
This description was started by a non-fighter who is drawing on his observations. Could someone please check this and edit the page appropriately.