Gaki is the Japanese translation (literally "hungry ghost"), of Preta, the Sanskrit name for a type of supernatural being described in Hinduism, Buddhism, and Taoism.
A Gaki can be either the soul of a person who was overly compulsive, deceitful, jealous or greedy during their life just like their Preta brethren or they can be a demonic spirit.
In either case the only religious items that can hold them at bay are those of Shintoism (and perhaps Buddhism) They can shapeshift (sometime taking the form of someone their target knows) preferring to slowly drain their prey to make it appear to be an illness. They can be destroyed by normal weapons but can take more damage then a human.
There are many "species" of Gaki each with it own diet; in addition to the Gaki that drink blood there are those that feed on incense, paper, sweat, or tea. Some can feed on a person's very thoughts as they meditate.