Venom is a mechanic where an animal or poisoned weapon can inflict (bio) Damage or other conditions for a set length of time. Venom is the main method for (bio) damage to be inflicted.
Envenomation does not necessarily mean that venom has been used. For example, the bullet-castes of Death Angel warriors, workers, Queens and turrets deal 5 (bio) damage per turn as they eat their foe from the inside. Machines cannot be envenomated. To envenomate a target, 50% or more damage has to be dealt through armour.