Server admins frequently need to clean up messy worlds. If you are searching for "what command kills all entities but you", "command to kill all items", or "how to kill all villagers minecraft", here is a rapid reference guide.
Kill ALL Dropped Items
To instantly delete every dropped item on the ground (great for fixing a laggy server where someone blew up a mountain of chests):
/kill @e[type=item]
Kill All Entities But NOT the Player
If you run /kill @e, you will kill yourself and every player on the server alongside all the cows, pigs, and zombies.
To wipe the slate clean of entities without touching the players, type:
/kill @e[type=!player]
(Note: This might also kill item frames and armor stands! Be careful in spawn areas!)
How to kill one type of entity in Minecraft
If a player accidentally spawned 5,000 villagers and crashed the economy ("how to kill all villagers minecraft"), you can specifically target them:
/kill @e[type=villager]
Targeting multiple entity types
If you want to know "how to use /kill to kill multiple types of entities", vanilla Minecraft doesn't natively allow a singular array command easily without datapacks. The easiest admin solution in-game is simply running the command back-to-back:
/kill @e[type=zombie]
/kill @e[type=skeleton]
/kill @e[type=creeper]
Keep your server's TPS high and your world clean!