Pets
If you are a class that uses pets you should learn everything you can about them, including how to best control them.

Summon Pet
Once you are allowed to have a pet you need to learn how to summon it. Typically, you need to first get training to receive your pet. Once you do that, a new summon pet button will appear in your spells menu. Hunters have to convert a pet from the wild with their special ability. Drag the summon pet button on to one of your action bar pages, and summon your pet. You will now see a new mini action bar that is used to control your pet. This is your pet action bar.

Hmmm...

Pet Action Bar
Pet users get a special, smaller action bar they can use to control their pet. You can drag abilities onto this action bar. Your pet bar disappears when your pet dies. You can modify the auto-cast status of pet spells in the pet spellbook.

Pet Training
You can gain new abilities for your pet by visiting a pet trainer for that specific pet. If you don't know where to find one, ask more experienced players of that class. The trainers are around, but they may be difficult to find. Try visiting a new town.

Pet Location
Your pet shows up on the mini-map. The mini-map is very useful for keeping track of how far away your pet is as you travel around the world.

Pet Death
If you get too far away from your pet, or if the pet gets stuck, it will eventually unsummon. You will have to summon your pet again. Learn to lead your pet around so it doesn't fall behind. Another option is to dismiss your pet, and then summon it again once you've reached your destination.

Pet Healing
Your pet will heal after combat is over. Also, you can heal it yourself.

Pet Buffs
You can buff up your pet! Give it a shot.

Dismiss Pet
Right-click on your pet's name bar to see additional pet options. Warlocks can dismiss a pet if they no longer need it or wish to switch pets. Hunters need to use an ability "dismiss Pet" to dismiss their pets.

Abandon Pet
Hunters can Abandon a pet forever in order to acquire a new one. If a less permanent solution is desired, they can stable a pet or use "dismiss Pet"

Stable Pet
Hunters can choose to stable their pet at an NPC which is usually located outside a local inn. Stabling a pet is like storing it in a bank, a less permanent solution than abandonment. Once the pet is stored, the hunter can tame and train a new pet which can also be stabled or unstabled at any time. Only 2 pets can be in a stable at any given time while a third is unstabled and summonable at the hunter's command.

Pet Modes
While your pet is active you have several common buttons to control it:
Attack: Select a target, and hit the attack button. There's a hot key to do it too. Look it up in the options/controls menu.

Follow: This command tells your pet to ignore everything else and just follow you.

Stay: This is useful when you want your pet to stay in place. You can leave it outside of town or a building, go do your business, and then come back to it. That way it won't be traveling around trying to reach you.

Aggressive: If a monster comes within range of the pet, the pet moves to attack the monster.

Defensive: The pet responds if attacked but otherwise won't act.

Passive: The pet just sits there and does not attack. This is useful if you want to stop a pet from attacking to play around with hate lists or for other reasons. It comes in handy.

Pet Control
You need to manage the hate of the monster between you and your pet. The monster attacking your pet is about to kill it then you need to either heal your pet or get the monster to attack you instead. You can transfer the monster over to you by casting spells on the monster that really make it angry. You will have to figure out which spells do that.
If the monster is too focused on you, you need to stop attacking it and use whatever pet abilities your pet has to make the monster angry enough to fight it instead. Learning how to properly manage monster hate can take some time. The goal is to keep both you and your pet alive until the end of combat.

If you're in a party and your pet is attracting too much attention, have it stop attacking until the monster focuses on someone else in the party. Then wait a bit longer and have your pet rejoin the fight again. That way your pet can continue to attack and live while the monster is attacking someone else.

When you give your pet commands it will cancel any spells they are currently casting.

Pet Stats
All pets receive a percentage of their master's armor, spell damage, spell resistances, attack power, and stamina. You can check out the bonuses on your character screen under your stats. For example increasing your Intellect will also increase your pet's Intellect as shown on the character screen under base stats.

Rookie Mistakes
If your pet is in Aggressive mode inside of a dungeon it will typically go crazy and wake up lots of monsters. Change it to Passive or Defensive mode.

Crikey!
When traveling, make sure your pet is not in Aggressive mode. Otherwise, your pet will really slow you down by attacking everything you pass. You should either change it to Passive mode or unsummon the pet if you don't think you will need it.

At the start of a battle make sure your pet is attacking. If it is standing around, it's not helping. You can use hot keys to make your pet attack faster. To make the pet attack, press Control and 1 buttons at the same time.

Pet Fun
Go into a newbie region and use your pet to destroy some monsters. It is funny to have your pet do all the work as you watch!
Kill someone with your pet in player vs. player combat!
Use emotes on your pet! /hug pet =]
Use powerful pets to scare newbies. They will think it's a monster if they are not familiar with the pets that players can use.

Other Information
Warlock and Hunter pets will be automatically dismissed when you summon a mount. These pets will return on dismount. This does not affect pet happiness for Hunters.
Pets will not attack targets that are under the influence of crowd control (sheep, banish, etc.).
Pets, summons and guardians are not susceptible to charm.
Pets and summons will use their combat abilities as often as possible.
If you tell your pet to attack a creature, it won't be distracted unless taunted.
The pet action bar is greyed out while the pet is confused or fleeing.
Pets of other party members display in the party UI. You can see pet health and harmful effects on the pet. You can turn this functionality on and off in the Interface Options menu.

Common Pet Questions
Can I give items to my pet?
No.
Can I name my pet?
Hunters can name their pets. Warlocks cannot.

Do pets take up extra experience?
No.

What classes get pets? Warlocks and Hunters can get permanent pets. Priests can control humanoid monsters as a pet for a limited amount of time. There are other ways to get pets. For example, Engineers can construct pets. There are items that players can find that create a pet for a limited amount of time. Players can also buy non-combative pets that are purely for show.