- looted from a corpse or container
- given as a quest reward from a NPC
- purchased from a vendor
- crafted by a player
- received through in-game mail
It is not possible to have an item that binds when picked up in your inventory without it being bound to you.
Most loot from bosses and rewards from quests are Bind on Pickup. Vendor-purchased PvP rewards and most reputation rewards are also Bind on Pickup. A warning appears before a player loots a Bind on Pickup item, asking if the player is sure they want to do so. This also applies for rolling on a BoP item (either Need or Greed) while using Group Loot. However, BoP items acquired while in a group can be traded to other group members if conditions are met. This is normally only available for a limited time; in the case of instance drops the items may be traded for up to two hours after they dropped. Enchanting, socketing or otherwise manipulating an item will make it immediately untradeable.
Crafted items that are Bind on Pickup can only be acquired by making them yourself, although it is sometimes possible to drop the crafting profession after having made any BoP items you want from it. However, some of these items require a certain skill level or specialization in order for you to equip them.
Note that the in-game tooltip actually displays the words "Binds when picked up", which is why some players use "BwP", but "BoP" is the most common form. The term "Bind on Acquire" is sometimes used by players, but never appears in-game and is unsuitable as its abbreviation (BoA) can easily be confused with the Bind to Account form of soulbinding.