Note, this is NOT for the siege weapons that share the same shape (ballistae).

Crossbows were infamous when they were invented since they made full plate armor absolutely useless, the bolt could easily penetrate the protective metal and fatally wound the person supposedly protected from all harm. This allowed a completely untrained peasant to trivially kill a knight who had years of experience behind them despite their heavy armor. However, modern body armor can block crossbow bolts due to their need to block bullets (though conversely modern armor tends to be quite useless against blunt weapons like hammers).

Related: (weapons)
* Bows

Related:
* Projectile physics
* Projectile recovery
* Archery (as the sport)

Similar:
* Speargun

Related: (pre-modern projectile weapons)
* Thrown weapons
* Slings