You got five Demon slots.
- One for small Demons that shoot fireballs (like Imps);
- One for sturdy tank Demons (like Voidwalkers);
- One for alluring Demons (like Succubi);
- One for magic eating Demons (like Felhunters);
- One for Demonic Musclemen (like Felguards);
And a “Demon Summoning” Class Profession (Like DK Runeforging).
You collect the Recipe for a specific Demon out in the world, gather the ingredients, draw the Summoning Circle in a specific place (Dreadscar Rift, maybe?), call forth their name, then you beat the Demon in single combat.
Once the Demon is beaten, a contract is established and they become your minion.
Each specific Demon is bound to a specific slot. For instance, Imps, Gan’arg and Wyrmtongues would all belong to the “Imp family”.
Trivia: Before Cataclysm erased all Class Quests, the Questlines to obtain each Warlock Demon worked roughly the same way as this, as far as fantasy goes. You would obtain the items necessary, go to a Summoning Circle, Summon and fight a Demon. Beating the Demon and completing the quest awarded you the new spell.