A drop is any item obtained from the environment, such as an item looted or "dropped" off of enemy mobs or found in chests. A drop is also known as loot.
Quest drop[]
A drop of a quest item. Many quest items only drop if the player currently has the associated quest in their Quest Log.
Most items that require a drop of only a single item, such as the retrieval of a specific item, or the removal of a head from a named mob, each player in the group that loots the mob can each loot one of the items.
Quest items that require you to collect a certain number of items can only be looted by one player at a time.
The former can be looted by any questholders regardless of loot priority (such as Group Loot or Free For All), the latter are looted as normal. If only one person has the quest to collect a number of items, they may loot from each mob that drops a quest item, regardless of loot priority. They will also be able to loot any money, and start any loot threshold rolling on items whose quality is at or above the loot threshold.
There are still other quest items that drop regardless of having the quest or not. Some of these have alternate uses, others are used only for the quest they are attached to. Often they are not soulbound items, and can be traded, sold or bought by and from players.
The final Quest Drop is the drop of an item that starts a quest. Each player in the group can loot the item, regardless of loot priority. The quest item may still have a level requirement, and some quest items of this nature can be used in other ways (e.g., Ghosthowl's Pelt may be used as a cloak).
World drop[]
A world drop is a drop that has a random chance to drop off any mob or chest (within a certain level range) anywhere in the world. World drops are largely impossible to farm for because no specific mob or group of mobs have a more than a minuscule chance of dropping the item. While most World Drops are usually a drop of uncommon quality (green) or better, this is not universal.
Example: [Recipe: Swiftness Potion] can drop off almost any mob between around level 12-20. However, it drops very rarely.
Example: [Weather-Beaten Journal] can drop from any school of fish.
Many sought-after bind on equip rare (blue) and even rarer epic (purple) weapons and armor are world drops. Because of their rarity, some world drops can fetch high prices at the auction house.
Area drops also exist which occur in a specific continent or zone.
Continent drop[]
Not a very widely used term, but some uncommon or better random drops are actually restricted by continent.
Example: All "Bristlebark" named set items ([Bristlebark Boots], [Bristlebark Bindings], etc.) can drop off any mob within a certain level range in Kalimdor (including instances) but not in the Eastern Kingdoms. On the other hand, all "Silver-thread" armor (like [Silver-thread Gloves]) only drop in Eastern Kingdoms.
Zone drop[]
Zone drops are like partial world drops. They have a small chance to drop off any enemy, but only within a specific zone, instance or area.
Each instanced dungeon has its own unique selection of rare zone drops. Like world drops, they have an extremely low drop rate, can drop from any mob in the zone, and are usually bind on equip.
Example: [Necrology Robes] is a rare drop off any enemy in Shadowfang Keep.
Static drop[]
A drop that only comes from a specific mob or group of mobs.
Example: The [Spellforce Rod] only drops from Venture Co. Surveyors in a certain area of Stranglethorn Vale.
Boss drop[]
A type of static drop that only comes from a boss mob in an instance. The best ones are usually Bind on Pickup.
Epic drop[]
A drop of an epic quality purple item inside or outside an instance. Most drop inside instances, but there are a few that drop outside.
Legendary drop[]
A drop of a legendary quality orange item inside or outside an instance. Most drop from the highest level raid instance bosses and then only occasionally.
Artifact drop[]
A drop of an artifact quality (brown goldish) item. There have been no artifact drops yet.