Answer:
Bayonets are blades that are specifically designed to be attached to the end of a rifle, turning the rifle into a type of 'polearm'. Most bayonets, however, can be used as a standard knife when not attached to a rifle, although some are permanent fixtures of the rifle and cannot be removed easily.
So basically, when the bayonet is attached to the rifle, it is used mainly as a thrusting weapon, although slashing certainly is possible.
When not affixed, it is used as a standard knife.