Vore, generally short for voraphilia, is best summarized as the fascination and/or eroticizing of the idea of one being swallowing another being, whole and (generally) alive. It is generally thought of as a more obscure fetish - at least, more so than, say, BDSM, although online, the community of people who are in some part interested in vore is of a considerable size for such a specific kink.
As a fetish, vore has its roots in both the furry community and giantess (or GTS) fetish. In both, the theme of predator/prey domination (or, in the case of GTS, femdom) made vore a logical extension of either fantasy. Furry art and fiction forms a sizeable part of the total body of vore material available online, and entire websites and message boards exist for the discussion of giantess fantasies, several of which do involve being eaten.
Vore can also be thought of as related to cannibalism, although voraphiles are often quick to distinguish between the two. It is, for the most part, a valid distinction- most vore, known as soft vore, is mostly bloodless, not overtly violent, and does not involve the mutilation of the prey. However, the distinction between hard vore, where the prey is painfully and bloodily destroyed, and cannibalism is a thin one, the distinction being one of the motivation of the predator character.
In addition, the most notable difference between vores and cannibals is that people attracted to vore are aware that their fetish is purely fantasy. While a small segment of the overall population of vores express an interest in the actual, literal consummation (no pun intended) of the act, most are aware that actually being swallowed alive would be horrific, not to mention painful. By comparison, cannibals, particularly those for whom the act of eating another human being is erotic, see the carrying out of the physical, painful, and fatal aspect of this consumption as an essential part of their arousal and titillation.
That said, very little psychological work has been done to study this fetish, although it would probably prove interesting- themes of domination and submission, return to the womb (especially in unbirthing or vaginal vore.), intimacy, Freud's death instinct, and others are heavily played into as part of vore. Many vores, however, are resistant to coming out of the closet about their kink, because it is so particularly bizarre. Vore's association with the furry community also makes it hard to accept, as furries themselves are viewed as horribly deviant to begin with.
However, to simply say that vores are all a bunch of crazy people running around in fursuits, biting each other would be an unfair generalization. While a large number of vores are furries, and express their sexuality through their furry characters, there are many others who have other interests- to stereotype a voraphile in this way would be like saying that all BDSM Master/Mistress & slave couples carry that aspect of their relationship into other areas of their lives, that all gay men are effeminate, or that all transvestites are gay. Some are, but some are not.
Vore artwork is particularly fascinating in its menagere of predator and prey characters, and for often times making what is general a horrific, terrifying act into something that some can't help but stop and observe. Browsing the online art gallery DeviantArt or the specifically vore-centric message board Eka's Portal can bring up a wide range of images, ranging from childish doodles to beautifully rendered, highly detailed illustrations of various stages and methods of vore. Whole fictitious universes have been built around vore as a common theme, the best known of which being Felarya, created by a French illustrator going by the name Karbo. Numerous writers and other young artists have contributed to this realm, which has its own Wiki at www.felarya.com.