White Court vampires are one kind of the various unrelated creatures called "vampires". White Court vampires appear human; they prey on human emotion and energies. Their corresponding organization is the White Court, after which they are named.
White Court vampires appear human and prey on human emotions and energies. They are long lived, possibly immortal; Lord Raith and Lara Raith have both lived for centuries. The ones we've seen can be identified by a shared inhuman beauty, and blood that is "a few shades too pale to be human".
Their vampiric state is genetic and highly hereditary, rather than created like the other two Courts. They are, for all intents and purposes, human until their vampirism manifests sometime around the age of their maturity. As with the Red Court, their first feeding, with its subsequent and almost inevitable kill, usually marks their full transformation, and awakens the demon that lives inside of them, forcing them to feed. If they refrain from feeding for too long, this "Hunger" drives them into a frenzy where they must feed, usually enough to permanently harm or kill their victim, and can drive the vampire permanently insane. However, under specific circumstances, individuals descended from White Court vampires may curtail their transformation into vampires completely.
Their demon feeds upon specific emotions, which largly depends upon which family you come from: House Raith normally feeds on lust, for example; House Malvora feeds on fear; House Skavis feeds on despair, and so on. They can change the emotion that they feed on, such as with Madrigal Raith, but it is considered very taboo. The feeding is addictive, and victims usually come back for more.
White Court vampires are not endowed with as much physical strength as their vampire counterparts under normal circumstances. Their physical performance levels (standing jump heights, speed, endurance, reflexes, rate of healing, and so on) are majorly dependent on how much their demon is fed. If they use too much of their supernatural powers, they must feed again to replenish them. While tapping into the demon, they radiate waves of cold (possibly an illusory reaction of normal humans to their energy sink), their skin whitens, their eyes turn an unnatural silver, and their whole body begins to glow faintly.
They have far fewer vulnerabilities than other kinds of vampires. Sunlight and symbols of faith do not harm them at all. Their largest weakness is the opposite to the emotion that the vampire primarily feeds upon: Raiths are harmed by true love, Malvora by acts of courage, Skavis by hope, etc. People or objects who are (or who have recently been) contaminated by the opposite emotion of the specific vampire are highly resistant to that vampire's control, and can even physically burn and blister the vampire's skin upon contact.
The Whites can cross a Threshhold without an invitation but doing so cripples their abilities.