According to Rupert Giles, an occult expert in the series, when an ancient race of demons called the Old Ones were banished from Earth, the last one fed on a human and mixed their blood, creating the first vampire.
On occasions, vampires would recruit humans such as criminals or services from occult organizations like Wolfram & Hart for tasks during daytimes due to their inability to be around sunlight.
The series vampires exhibit superhuman abilities, including increased strength, heightened senses, and accelerated healing.
Vampires can change at will between human appearance and a monstrous form with a pronounced brow ridge, yellow eyes, and sharp teeth.
The vampires in Buffy can be killed when they're beheaded, exploded, penetrated through the heart by a stake (wooden object), or come into contact with sunlight, fire, or excessive amounts of holy water.
Spike, however, is shown on several occasions driving during the day by blocking the sunlight coming through his car windows with foil, and leaving a small strip clear to see the road.
Vampires' flesh burns upon contact with blessed objects such as holy water, a Bible, recently consecrated ground, or a Christian cross.
Vampires generally cannot reproduce sexually, but mystical circumstances allow the characters Angel and Darla to conceive a son, Connor, who has a human soul with vampire-like abilities of strength, endurance, agility, and senses, but none of their weaknesses or need to drink blood.
However, soulless vampires are capable of feeling human emotions such as love, though these tend to be expressed as twisted and obsessive behavior, and without souls, they typically can only experience the negative affectivity associated with evil.
[7] The character of the Master was designed to be in permanent vamp face to highlight his age and make him appear animalistic.
Make-up artist John Vulich based the Master's appearance on a bat, saying that the character has devolved to a more primal, demonic state over the years.
[9] Each episode of Buffy the Vampire Slayer contains fifty to eighty special effects shots around 10 percent of that ended up in the final shows.
Whedon said that whereas shows such as The X-Files spend time explaining the science behind the supernatural and making it as real as possible, Buffy and Angel are more concerned with the emotion resulting from these creatures and events than justifying how they could conceivably exist.
The shows therefore tend to gloss over the details of vampire and demon lore, simply using the Hellmouth as a plot device to explain unexplainable things.
Joss Whedon felt this concept was a "charming notion" but rejected it because he believed that if every vampire in the show were dressed in old-fashioned clothes, they would cease to be scary.