Due to the fact that World of Warcraft Warlock gets an amount of pets that have different roles. Getting to know which role each pet plays can help Warlock pick the right pet in any situation.
   The first pet a Warlock obtains is the Imp, a damage pet. The imp pet is a ranged attacker and be completed at level 2. The imp quest takes little time and once the player completes it, the summon imp spell is added to the spell book of the warlock once it is completed. The imp does not have a lot of hit points and is the most fragile of the warlock pets.
The Imp is best used when tanking or crowd control ability is not needed. The imp adds DPS, but from a grouping standpoint, the most important ability of the imp warlock pet is bloodpact.. Bloodpact boost the hit points of all members of the group. A fire shield can also add extra damage, although the warlock should not set his imp to cast fire shield automatically.
The second pet for Warlock is Voidwalker. It will be the primary solo pet used by the warlock. The only time groups will ask for it specifically is when no other tank is available. In most situations the voidwalker pet will hold the attention of the mob while the warlock stands back and casts spells. The grimoires available for the pet are all centered on the idea of the void walker keeping agro or emergency abilities used to keep the master of the voidwalker alive. The voidwalker is the only pet with the ability to heal itself between battles
When reaches level 10, a Warlock will need to return to a capital city to get the void walker quest. This may require the player to travel a little or a lot in the gnome variant of the quest. All of the warlock pet quests after the imp quest require the warlock to summon a demon and defeat it in the summoning circle once the required components are gathered.
The third pet is the Succubus, a Crowd Control Pet. Warlock will return to the person who gave him the voidwalker pet quest and be given another quest to obtain a succubus at level 20. The succubus does higher damage then the voidwalker, but has fewer hit-points. The succubus's main strength is its crowd control abilities. In the absence of a Mage in a group, the succubus pet's soothe ability can keep a mob out of combat while the party members and the warlock take care of more immediate threats.
The last pet is the Felhunter pet. The total hit-point of the felhunter is closest to the hit point total of the voidwalker, but it lacks tanking ability. The stun/drain ability of the felhnuter pet make it invaluable to warlocks who engage in player versus player combat, as it keeps the other casters from casting. The felhunter has limived use in a player versus environment setting, but may still find use in areas where a lot of caster mobs dominate. The Felhunter can also be used to remove harmful spells on friendly targets or strip beneficial spells from unfriendly targets.
For both Alliance and Horde characters, the Felhunter quest takes place in Ratchet, the neutral goblin city. Alliance and Horde characters have a different starting point, but it will end up in the same place.
Before starting an instance which pet you prefer to choose, it is better for you to ask a group. In many cases, the above mentioned pets may be required depend on the situation. Knowing which pet is best suited to a given situation is helpful for Warlock more or less.