First off you must have the Entity Id exactly the way shown; for example, /summon Cow <Must have a capital letter at beginning to be summoned

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First to start you can summon anything. This is the basic use of the command

/summon [Entity] [x] [y] [z] [DataTag]

First off the [Entity] is what you summon. What you have here is the Entity (Cow,MushroomCow,EnderDragon,EntityHorse,Sheep)

Now the [x] [y] [z] part is the coordinates. You can put ~ ~ ~ to summon the Entity right where you are.

Next the [DataTag] is the most complicated part.

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Now there is one mob you can spawn only using the datatag: Charged Creeper. You can do that by /summon Creeper ~ ~ ~ {powered:1}
Next you can equip mobs with items:
/summon Zombie ~ ~ ~ {Equipment:[{id:276}, {id:306}]}
That would spawn a zombie equiped with a diamond sword and iron boots.

Next you can make an invincible mob by doing the following:
/summon Cow ~ ~ ~ {Invulnerable:1}

To change the health of the entity you are spawning do the following:
/summon Sheep ~ ~ ~ {Atributes:[{Name:generic.maxHealth, Base:50.0}]}
This would summon a sheep with 50 health (25 hearts).

The last thing I am going to talk about before you leave is the Riding DataTag:
/summon Enderman ~ ~ ~ {Riding:{id:"EnderDragon"}} (My personal favorite :D)

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Thank you for reading! I learned most of this from Minecraft Forums.