Main usage/Commands:



@p = player
@a = all players
@r = random player
@f = player farthest away


Command Blocks can also be used with the new /gamerule command. The /gamerule command changes the rules of the game. It can be used to turn fire spread on and off, to change whether or not mobs spawn, blocks drop, whether a player can keep their inventory after death and others.


A quick guide to minecraft /gamerule commands:


commandBlockOutput o Changes whether or not the command block outputs text to the console, ie, whether the player will be notified as to changes of game rules or not.
doFireTick o Turns fire spread on or off.
doMobLoot o Turns mob drops on or off.
doMobSpawning o Turns mob spawning on or off.
doTileDrops o Turns block drops off (ie, if you break a block, it will or will not drop and be able to be picked up.)
keepInventory o Decides whether a player can keep their inventory after death.
mobGriefing o Turns the ability of mobs to destroy blocks through creeper explosions or endermen grazing.

/give (player name) (id) (ammount) (data/for wool, potions, etc.)

/tp (player) (x y z/player)

/say (somhening)

/clear (player name)

/gamemode (survival,adventure,creative) (player name)

/gamerule (gamerule) (true/false)

/spawnpoint (player name) (x y z)

/toggledownfall

/weather (rain, clear, thunder, etc.)

/xp (amount) @p -give xp to the nearest player or /xp (amount) (player name)

New commands:

/spreadfire off/on/ --- pretty self explanatory, turns off the ability for fire to "spread" from one block to another in that world

/naturalspawn on/off - toggles the natural spawning of a specified monster (natural spawning = any non mob egg/spawn cage spawn), so "/naturalspawn creeper off" would prevent creepers spawning in an area a distance away with the correct light level.

/explosionblockbreak on/off - explosions (ghast fireballs, creepers, wither hits, tnt, beds in the nether) will explode and damage players but not break any blocks

/icemelt on/off - ice will not melt, regardless of light level around it.

/crafting on/off - toggles whether players can craft items in their 2x2 inventory grid or a 3x3 table, useful if crafting items could bypass an intended section of a map

/tradechange on/off - toggles whether or not villager trades will appear or dissapear(or have a chance to) after use

Source: Minecraft Wikki, SethBling

If you have problem with setting the commands just watch SethBling's video: www.youtube.com/watch?v=e5ww0c8wxzs

PS: I'm not sure if the /explosionblockbreak command works.