The butchering profession in Warhammer Online is a trade skill that retrieves resources from dead enemies, specifically dead animal mobs. The resources that are retrieved from the dead mobs can then be used for the apothecary profession. But before getting into the detail theres a few guidelines or rules to be aware of before jumping into butchering. Any corpse of a non-sentient creature that has been fully looted can be butchered. When a corpse is butchered the loot produced and obtained is dependent on your skill level, that is the higher skill level the better the loot. Note though that a mob won't produce any loot that is higher than its level. When a corpse is butchered a skill-up is dependent on the player's skill level versus the mob's level and finally the chance of a skill up does down gradually as the skill approaches 200. Also items drop according to mob type. That is no matter the level of the mob, a wolf will always produce nutrient and ingredients for restoration potions. The caveat is that the higher level the wolf the higher level the material produced from butchering.
So why pick butchering? Butchering provides a great amount of material for the apothecary profession including main, secondary, and fertilizing ingredients, cutting down the player's expenses in producing potions. Second, with any gathering profession but especially butchering the player can turn quite a profit selling the materials gathered via the auction house. Third, materials from butchering are very easy to acquire. Kill the required mob, loot, butcher, and repeat. Thus farming specific or profitable ingredients can be very simple, just hang around where that mob spawns, kill and butcher it, then repeat.
Butchering trainers can be found in chapter two. Trainers for the Order can be found in Gray Mare Inn and Adunei. Trainers for Destruction can be found in Akrana's Storm and Da War Maker.