The exact nature of any given herd varies from vampire to vampire, but whatever the circumstances, these mortals relish the ecstasy of the vampire's "kiss". Some vampires foster cults around themselves, setting themselves up as a god to be worshiped or placated with a tribute of blood. Such cults are rarer in the modern era than they once were, when death cults and mystery cults (like that of Mithras in Vampire: the Masquerade lore) could draw mortals into their clutches with relative ease. Others use connections within the medical community to gain access to easy sources of blood, and so on. In the modern nights, many herds draw from the throng of blood dolls that accumulate in the wake of Kindred activity.
While having a herd can be a great benefit to a vampire, providing easy access to food and freeing up his nights for more pressing matters, it is sometimes a weakness. Herds need to be protected from rival vampires, and their members are useful for little other than serving as a vessel for blood. Occasionally, a mortal from a herd might prove themselves worthy of becoming a ghoul or even of the Embrace, but most Kindred consider them sources of food and nothing more.
By default, it is assumed that all members of a vampire's herd are standard mortals, although some might be (or might have once been) wholly unexceptional ghouls; in Vampire: the Requiem, these ghouls would almost certainly be suffering from blood addiction.
In Vampire: the Masquerade, Herd is a Background; in Vampire: the Requiem, Herd is a Merit. Any members of a herd that happened to be exceptional would be mechanically treated as Retainers or even Allies, even if they would be considered part of their master's herd by their master or other Kindred.