The pact begins with a blood signature on a contract. Associates have been working with witches since the Cataclysm, a partnership for the sake of power.
Witches can be any species as long as they have a soul to spare for their soon-to-be familiar. They're one of the few living beings who live without one. In exchange, they can wield magical powers which can be difficult to control without ritual tools (wands, cards, etc.)
Associates on the other hand are their own creatures. They start out small and round and have to build their bodies on their own with their surroundings; their size directly correlates to their power. When they die, their children are born using their body parts. While they provide the magic, compared to their witches, they can't channel it as effectively as the witches.
If a witch were to have a child (known as a witchspawn), the child would inherit their powers without the need for a pact. Their magic tends to be weaker since there's no proper way to develop them.
The pact ends when the witch dies or if they revoke the pact. If revoked, the associate dies. And with their death, the associate walks away to form another.