At its simplest, a longspear is little more than a sharp pointed blade mounted on the end of a 10-foot-long pole. A longspear has reach. You can strike opponents 10 feet away with it, but you can't use it against an adjacent foe. If you use a ready action to set a longspear against a charge, you deal double damage on a successful hit against a charging character.
Simple Weapon - Melee
Two-handed weapon. This weapon has reach; you can use it to strike foes 10 feet away, but not adjacent foes. This weapon deals double damage when readied against a charge.