All of the commands are (as of 1.7.10 Vanilla):
/gamemode (number)
It changes your gamemode, 0 is survival, 1 is creative, 2 is adventure, and 3 is spectator.
/tp (player) (player) or /tp (player) (coordinates)
Teleports you to a player.
/kill (player)
Kills anything you desire.
/difficulty (peaceful, easy, medium, hard)
Selects a difficulty.
/xp (amount) (player) or /xp (amount)L (Player)
Gives a player a certain amount of experience.
/help
Lists all of the commands.
/time (set:add) (value)
Adds or sets a time.
/weather (clear; rain; thunder) (duration in seconds)
It changes the weather for a certain amount of time or randomly if a duration is not selected.
/clone (x1) (y1) (z1) (x2) (y2) (z2) (x) (y) (z) (mode)
Copies anything from x1, y1, and z1, from x2, y2, and z2 to a set of coordinates.
/execute (entity) (coordinates) (command)
Executes a command from an entity.
/achievement give (stat name) (player)
Unlocks an achievement for a player.
/testforblock (coordinates) (tile name) (data value) (data tag)
Sees if a certain desired block is in the coordinates specified.
/blockdata (coordinates) (data tag)
Changes the data tag of a holder block to whatever specified.
/msg (player) (message)
Messages a player a private text.
/spawnpoint (player)
Sets a person's spawn point to where they are standing so that when they die they will respawn there.
/setblock (coordinates) (tile name) (data value) (old block handling) (data tag)
Places a specific block down where the coordinates specify.
/toggledownfall
Toggles the weather from rain to sunny or from sunny to rain.
/say (message)
Broadcasts a message.
/testforblocks (x1) (y1) (z1) (x2) (y2) (z2) (x) (y) (z) (mode)
Similar to /testforblock except that it tests for multiple blocks in the x1, y1, and z1 to the x2, y2, and z2.
/summon (entity) (coordinates) (data tag)
(See our "The Summon Command" page.)
/?
See /help.
/give (player) (item) (amount) (data) (data tag)
Gives a player a specific amount of items of a specified type.
/scoreboard (objectives;players;teams)
Sets a scoreboard; records of something specified in the three categories.
/spreadplayers (x) (z) (distance) (max range) (respected teams true;false) (player)
Teleports or spreads a player a certain amount of blocks away.
/gamerule
Sets a gamerule that changes gameplay.
/clear (player) (item) (data) (amount) (data tag)
Clears a certain items and amount from a player.
/particle (particle name) (coordinates) (x distance) (y distance) (z distance) (speed) (count) (player;entity)
Shows a particle effect of some sort at a specific area.
/tell (player) (message)
See /msg.
/me (message)
Broadcasts a message but in a different way than /say
/trigger (objective) (add;set) (value)
Changes a scoreboard objective value.
/seed
Broadcasts what seed is being used in the world.
/tellraw (player) (raw json message)
Tell a player a coded message.
/fill (x1) (y1) (z1) (x2) (y2) (z2) (tile name) (data value) (old block handling) (data tag)
Similar to the /setblock command except it fills a range of blocks.
/playsound (sound) (player) (coordinates) (volume) (pitch) (minimum volume)
Plays a sound to a specific player from specific coordinates.
/w (message)
See /msg.
/effect (player) (effect) (seconds) (amplifier) (hide particles)
Gives a certain player an effect for a certain amount of time.
/testfor (player) (data tag)
Searches for a specified player.
/enchant (player) (enchantment id) (level)
Enchants an item in a specified player's hand.
/gamemode (number)
It changes your gamemode, 0 is survival, 1 is creative, 2 is adventure, and 3 is spectator.
/tp (player) (player) or /tp (player) (coordinates)
Teleports you to a player.
/kill (player)
Kills anything you desire.
/difficulty (peaceful, easy, medium, hard)
Selects a difficulty.
/xp (amount) (player) or /xp (amount)L (Player)
Gives a player a certain amount of experience.
/help
Lists all of the commands.
/time (set:add) (value)
Adds or sets a time.
/weather (clear; rain; thunder) (duration in seconds)
It changes the weather for a certain amount of time or randomly if a duration is not selected.
/clone (x1) (y1) (z1) (x2) (y2) (z2) (x) (y) (z) (mode)
Copies anything from x1, y1, and z1, from x2, y2, and z2 to a set of coordinates.
/execute (entity) (coordinates) (command)
Executes a command from an entity.
/achievement give (stat name) (player)
Unlocks an achievement for a player.
/testforblock (coordinates) (tile name) (data value) (data tag)
Sees if a certain desired block is in the coordinates specified.
/blockdata (coordinates) (data tag)
Changes the data tag of a holder block to whatever specified.
/msg (player) (message)
Messages a player a private text.
/spawnpoint (player)
Sets a person's spawn point to where they are standing so that when they die they will respawn there.
/setblock (coordinates) (tile name) (data value) (old block handling) (data tag)
Places a specific block down where the coordinates specify.
/toggledownfall
Toggles the weather from rain to sunny or from sunny to rain.
/say (message)
Broadcasts a message.
/testforblocks (x1) (y1) (z1) (x2) (y2) (z2) (x) (y) (z) (mode)
Similar to /testforblock except that it tests for multiple blocks in the x1, y1, and z1 to the x2, y2, and z2.
/summon (entity) (coordinates) (data tag)
(See our "The Summon Command" page.)
/?
See /help.
/give (player) (item) (amount) (data) (data tag)
Gives a player a specific amount of items of a specified type.
/scoreboard (objectives;players;teams)
Sets a scoreboard; records of something specified in the three categories.
/spreadplayers (x) (z) (distance) (max range) (respected teams true;false) (player)
Teleports or spreads a player a certain amount of blocks away.
/gamerule
Sets a gamerule that changes gameplay.
/clear (player) (item) (data) (amount) (data tag)
Clears a certain items and amount from a player.
/particle (particle name) (coordinates) (x distance) (y distance) (z distance) (speed) (count) (player;entity)
Shows a particle effect of some sort at a specific area.
/tell (player) (message)
See /msg.
/me (message)
Broadcasts a message but in a different way than /say
/trigger (objective) (add;set) (value)
Changes a scoreboard objective value.
/seed
Broadcasts what seed is being used in the world.
/tellraw (player) (raw json message)
Tell a player a coded message.
/fill (x1) (y1) (z1) (x2) (y2) (z2) (tile name) (data value) (old block handling) (data tag)
Similar to the /setblock command except it fills a range of blocks.
/playsound (sound) (player) (coordinates) (volume) (pitch) (minimum volume)
Plays a sound to a specific player from specific coordinates.
/w (message)
See /msg.
/effect (player) (effect) (seconds) (amplifier) (hide particles)
Gives a certain player an effect for a certain amount of time.
/testfor (player) (data tag)
Searches for a specified player.
/enchant (player) (enchantment id) (level)
Enchants an item in a specified player's hand.