The Warlock is able to summon several different companions that loyally serve him. Only one pet at a time can be summoned though. Some companions are better suited for certain jobs than others. The Warlock need a Soul Shard for the summoning of almost every companion. One should exactly know which companion is going to be needed, so you don't end up with the wrong pet.

The first companion available is the Imp. While it isn't really a tough pet it is able to deal some respectable damage, especially in combination with the right talents. This is the only companion with a ranged attack and useful for situations where a melee pet would be in danger.

The second pet a Warlock will be able to summon is the Voidwalker. This pet is able to sustain lots of damage while keeping the enemies focused on himself. This makes him the best tanking pet, but its uses are limited, and you probably can't tank any dungeon or raid.

The Succubus is rather fragile as well, but is able to deal considerable melee damage. Additionally she is able to Seduce enemies, making them unable to fight.

The Fel Hound is the Affliction Warlock's favored pet. This pet is a melee pet, too, his qualities lie with his special powers though. He is able to interrupt spellcasting and can remove magic effects from friend and foe alike.

Finally there is the Fel Guard. This pet can only be summoned with the right talent, but it is that much more powerful and tougher than other pets. It deals way more melee damage than any other pet and is able to take a beating - although not quite as much as the Voidwalker.