/execute @e[type=ThrownEnderpearl] ~ ~ ~ fill ~-3 ~-1 ~-1 ~3 ~1 ~1 stained_hardened_clay 3 replace stained_hardened_clay
Executes enderpearls the action to fill a parallelepipedal area around it, but only replacing other kinds of clay (replace). If you want to change the color of the clay press f3+h then use the number after the slash [example: light blue stained clay (#0159/3)]

/execute @e[type=ThrownEnderpearl] ~ ~ ~ fill ~-1 ~-1 ~-3 ~1 ~1 ~3 stained_hardened_clay 3 replace stained_hardened_clay
Does the same of the previous, but in another direction (used to make the circular effect)

/execute @e[type=ThrownEnderpearl] ~ ~ ~ fill ~-1 ~-3 ~-1 ~1 ~3 ~1 stained_hardened_clay 3 replace stained_hardened_clay
Does the same of the previous, but in another direction (used to make the circular effect)

/entitydata @e[type=ThrownEnderpearl] {ownerName:""}
Changes the owner of the ender pearl (it's the name of the player that launched it, used to know which player teleport)

/execute @e[type=ThrownEnderpearl] ~ ~ ~ effect @e[type=!Player,r=3] 20 1 255 true
Executes ender pearls the action to give wither effect to all the entities that aren't players (!Player, the "!" string means all the entities but the one specified)

/execute @e[type=ThrownEnderpearl] ~ ~ ~ execute @e[type=!Player,r=3,rm=1] ~ ~ ~ particle dripWater ~ ~ ~ 0.5 1 0.5 1 100
Executes ender pearls the action to make at all the entities in the range of more than a block (rm=1) and less than three blocks (r=3) coords drip water particle

/execute @e[type=Snowball] ~ ~ ~ fill ~-5 ~-5 ~-5 ~5 ~5 ~5 stained_hardened_clay 0 replace stained_hardened_clay
It's the same of the first, but without the circular effect and the action is executed by a snowball

/scoreboard objectives add sa dummy
Adds a scoreboard called "sa" that can only be changed using commands (dummy)

/execute @e[type=Skeleton] ~ ~ ~ scoreboard players set @e[type=Arrow,r=3] sa 1
Executes all the skeletons the action to set the scoreboard of all the arrows in the range of three blocks (r=3) to 1

/execute @e[type=Arrow,score_sa=1] ~ ~ ~ fill ~-3 ~-1 ~-1 ~3 ~1 ~1 stained_hardened_clay 14 replace stained_hardened_clay
Is the same of the very first command, but it executes the action to an arrow with the score setted to 1

/execute @e[type=Arrow,score_sa=1] ~ ~ ~ fill ~-1 ~-1 ~-3 ~1 ~1 ~3 stained_hardened_clay 14 replace stained_hardened_clay
Is the same of the very first command, but it executes the action to an arrow with the score setted to 1

/execute @e[type=Arrow,score_sa=1] ~ ~ ~ fill ~-1 ~-3 ~-1 ~1 ~3 ~1 stained_hardened_clay 14 replace stained_hardened_clay
Is the same of the very first command, but it executes the action to an arrow with the score setted to 1

/execute @e[type=ArmorStand,name=flame,score_c_min=60] ~ ~ ~ summon FallingSand ~ ~ ~ {TileID:51,Time:1,Motion:[0.0,1.0,0.0]}

The execute command makes an entity (in this case an armor stand "type=ArmorStand", with 'flame' as name "name=flame" and with a score called 'c' settled at 60 or more "score_c_min=60") summon a falling sand in his position (~ means the relative position of an entity) with the id of a fire block ("TileID:51", 51 is fire's numeric id) and with an up-motion ("Motion:[0.0,1.0,0.0]", each 0.0 represents a relative coordinate-motion).

/scoreboard players add @e[type=ArmorStand,name=flame] c 1
adds one ("1") to the scoreboard of all armor stands (type=ArmorStand) called flame (name=flame)

/scoreboard players set @e[type=ArmorStand,name=flame,score_c_min=61] c 0
sets to 0 the scoreboard of all the armor stands ("type=ArmorStand") called flame ("name=flame") with the scoreboard settled to 61 or more ("score_c_min=61")

/execute @e[type=ArmorStand,score_c_min=1,score_c=1] ~ ~ ~ scoreboard players set @e[type=FallingSand,r=1] c 1
makes all the armor stands ("type=ArmorStand") with the scoreboard called c settled to 1 ("score_c_min=1,score_c=1") set the scoreboard 'c' of all the falling sands in the range of a block to 1

/execute @e[type=FallingSand,score_c=1] ~ ~ ~ kill @a[r=1]
makes all the falling sands with the score 'c' set to 1 kill all the players in the range of a block ("r=1")

/scoreboard objectives add c dummy
adds a scoreboard (c) which can be modified only with commands

/summon ArmorStand ~ ~-3 ~ {NoGravity:1b,Small:1b,Invisible:1b,CustomName:"flame"} 
summons an armor stand three blocks under the command block (~ represents relative coords). This armor stand doesn't fall ("NoGravity:1b") is small ("Small:1b"), is invisible ("Invisible:1b") and is named flame ('CustomName:"flame"')
PLACE THIS COMMAND THREE BLOCKS HIGHER THAN WHERE YOU WANT TO PLACE THE FLAME-GENERATOR