do you want to know hw to use command blocks like a pro?  this is the blog for you. this tutorial is for any on.

what is a command block?
a command block is a special block used to execute commands. you can do many thigs with command blocks.

how do i obtain a command block?
you can't obtain a command in survival mode without cheat,they are uncraftable.
to get them you do /give PlayerNamme minecraft:command_block
 
what can a command block do?

like i said before a command block executes commands.

examples of commands
here are some commands you can do
/summon Mob_Name ex: /summon Creeper or /summon ArmorStand ~ ~ ~ {ShowArms:1,CustomName:"Bob",customNameVisible:1,Riding:{id:"EnderDragon"}}
/kill 
/give PlayerName minecraft:Item Ex: /give Bill minecraft:command_block
/tell Player ex: /tell bill hi
/help
/scoreboard teams list 
injoy