/execute command

execute <player|entity> <x> <y> <z> <command>
execute <player|entity> <x> <y> <z> detect <x2> <y2> <z2> <block> <data> <command>

► Replace the block beneath a snowman with a packed ice block.

/execute @e[type=SnowMan] ~ ~ ~ setblock ~ ~-1 ~ minecraft:packed_ice

► Create heart particles above all horses only if the horse is 10 or more blocks away from where the command is executed.

/execute @e[type=EntityHorse,rm=10] ~ ~ ~ particle heart ~ ~4 ~ 1 1 1 1 3 force

► Create a creeper firework when a player is near a creeper.

/execute @e[type=Creeper] ~ ~ ~ execute @p[r=3] ~ ~ ~ summon FireworksRocketEntity ~ ~ ~ {LifeTime:25,FireworksItem:{id:401,Count:1,tag:{Fireworks:{Explosions:[{Flicker:1,Type:1,Colors:[4304440,3491355],FadeColors:4304440]},{Type:3,Colors:[10133921]}]}}}}

► Create a lightning bolt whenever a pig is detected on soul sand

/execute @e[type=Pig] ~ ~ ~ detect ~ ~-1 ~ minecraft:netherrack 0 summon LightningBolt

Notes:
You can join multiple execute commands and specify different player or entity conditions. For example, execute @e[type=!Chicken] ~ ~ ~ execute @e[type=!Bat] ~ ~ ~ command

More information:
http://minecraft.gamepedia.com/Commands