Here are the classes, structs, unions and interfaces with brief descriptions:

[detail level 123456]

NMinecraft
NServer
NFourKit
NBlock
CBlock	Represents a block. This is a live object, and only one Block may exist for any given location in a world
NCommand
CCommand	Represents a Command, which executes various tasks upon user input
CCommandExecutor	Represents a class which contains a single method for executing commands
CCommandSender	Represents something that can send commands and receive messages
CConsoleCommandSender	Represents the server console as a command sender
CPluginCommand	Represents a Command belonging to a plugin
NEntity
CDamageable	Represents an Entity that can take damage and has health
CEntity	Represents a base entity in the world
CHumanEntity	Represents a human entity in the world (e.g. a player)
CItem	Represents a dropped item on the ground
CLivingEntity	Represents a living entity in the world that has health and can take damage
COfflinePlayer	Represents a player identity that may or may not currently be online
CPlayer	Represents a player connected to the server
NEvent
NBlock
CBlockBreakEvent	Called when a block is broken by a player
CBlockEvent	Represents a Block-related event
CBlockExpEvent	An event that is called when a block yields experience
CBlockPlaceEvent	Called when a block is placed by a player
CSignChangeEvent	Called when a sign is changed by a player
NEntity
CEntityDamageByEntityEvent	Called when an entity is damaged by an entity
CEntityDamageEvent	Stores data for damage events
CEntityDeathEvent	Thrown whenever a LivingEntity dies
CEntityEvent	Represents an Entity-related event
CPlayerDeathEvent	Thrown whenever a Player dies
NInventory
CInventoryClickEvent	This event is called when a player clicks a slot in an inventory
CInventoryEvent	Represents a player related inventory event
CInventoryInteractEvent	An abstract base class for events that describe an interaction between a HumanEntity and the contents of an Inventory
CInventoryOpenEvent	Called when a player opens an inventory. Cancelling this event will prevent the inventory screen from showing
NPlayer
CPlayerChatEvent	Fired when a player sends a chat message
CPlayerDropItemEvent	Fired when a player drops an item from their inventory. If cancelled, the item will not be dropped and the player keeps it. The dropped item can be modified by plugins
CPlayerEvent	Base class for events related to a FourKit.Player
CPlayerInteractEntityEvent	Represents an event that is called when a player right clicks an entity
CPlayerInteractEvent	Called when a player interacts with an object or air
CPlayerJoinEvent	Called when a player joins a server
CPlayerKickEvent	Fired when a player is kicked from the server. If cancelled, the kick will not take place and the player remains connected. Plugins may modify the kick reason and the leave message broadcast to all online players
CPlayerMoveEvent	Fired when a player moves. Plugins may modify the destination or cancel the movement entirely
CPlayerPickupItemEvent	Thrown when a player picks an item up from the ground. If cancelled the item will not be picked up
CPlayerPortalEvent	Called when a player is about to teleport because it is in contact with a portal
CPlayerQuitEvent	Fired when a player disconnects from the server. Plugins may read or modify the quit message that is broadcast to all online players
CPlayerTeleportEvent	Holds information for player teleport events
CCancellable	Interface for events that can be cancelled by a plugin. When cancelled, the server will skip the default action
CEvent	Base class for all events dispatched by the server
CEventHandlerAttribute	Marks a method inside a Listener as an event handler. This class is not named "EventHandler" due to a naming conflict with the existing System.EventHandler
CListener	Simple interface for tagging all EventListeners Register instances with FourKit.addListener(Listener)
NInventory
CBeaconInventory	Represents the inventory of a Beacon. Single slot at index 0
CDoubleChestInventory	Represents the inventory of a Double Chest. 54 slots total — left side is slots 0–26, right side is slots 27–53
CEnchantingInventory	Represents the inventory of an Enchanting Table. Single slot at index 0
CFurnaceInventory	Represents the inventory of a Furnace. Slot layout: 0 = smelting input, 1 = fuel, 2 = result
CHorseInventory	Represents the inventory of a Horse. Slot layout: 0 = saddle, 1 = armor, 2+ = chest slots
CInventory	Represents an inventory containing items. Behavior relating to Material.AIR is unspecified
CInventoryHolder
CInventoryView	Represents a view linking two inventories and a single player (whose inventory may or may not be one of the two)
CItemStack	Represents a stack of items
CPlayerInventory	Represents a player's inventory, including armor slots and the held item
NNet
CInetAddress	Represents an Internet Protocol (IP) address
CInetSocketAddress	Represents an IP Socket Address (IP address + port number)
NPlugin
CServerPlugin	Base class that every plugin must extend
CLocation	Represents a 3-dimensional position in a world
CWorld	Represents a world, which may contain entities, chunks and blocks