Hi Snowprincess!
Let me answer your questions in the order your asked them:
1. Vampire, is it true? What is a vampire?
Vampires are mythical or folkloric creatures, typically held to be the re-animated corpses of human beings and said to subsist on human and/or animal blood (hematophagy), often having supernatural powers, heightened bodily functions, and/or the ability to physically transform. Some cultures have myths of non-human vampires, such as demons or animals like bats, dogs, and spiders. Vampires are often described as having a variety of additional powers and character traits, extremely variable in different traditions, and are a frequent subject of folklore, cinema, and contemporary fiction.
Vampirism is the practice of drinking blood from a person/animal. Vampires are said to mainly bite the victim's neck, extracting the blood from the carotid artery. In folklore and popular culture, the term generally refers to a belief that one can gain supernatural powers by drinking human blood. The historical practice of vampirism can generally be considered a more specific and less commonly occurring form of cannibalism. The consumption of another's blood (and/or flesh) has been used as a tactic of psychological warfare intended to terrorize the enemy, and it can be used to reflect various spiritual beliefs.
In zoology and botany, the term vampirism is used to refer to leeches, mosquitos, mistletoe, vampire bats, and other organisms that prey upon the bodily fluids of other creatures. This term also applies to mythic animals of the same nature, including the chupacabra.
2. If not, why are so many stories about it still surface? Or why do people believe them?
In his book, De masticatione mortuorum in tumulis, Michaël Ranft makes a first attempt to explain folk's belief in vampires. He says that, in the event of the death of every villager, some other person or people - much probably a person related to the first dead - who saw or touched the corpse, would eventually die either or some disease related to exposition to the corpse or of a fenetic delirium caused by the panic of only seeing the corpse. These dying people would say that the dead man had appeared to them and tortured them in many ways. The other people in the village would exhume the corpse to see what it had been doing. He gives the following explanation when talking about the case of Peter Plogojowitz: "This brave man perished by a sudden or violent death. This death, whatever it is, can provoke in the survivors the visions they had after his death. Sudden death gives rise to inquietude in the familiar circle. Inquietude has sorrow as a companion. Sorrow brings melancholy. Melancholy engenders restless nights and tormenting dreams. These dreams enfeeble body and spirit until illness overcomes and, eventually, death."
Nowadays, some people argue that vampire stories might have been influenced by a rare illness called porphyria. The disease disrupts the production of heme. People with extreme but rare cases of this hereditary disease can be so sensitive to sunlight that they can get a sunburn through heavy cloud cover, causing them to avoid sunlight — although it should be noted that the idea that vampires are harmed by sunlight is largely from modern fiction and not the original beliefs. Certain forms of porphyria are also associated with neurological symptoms, which can create psychiatric disorders. However, the hypotheses that porphyria sufferers crave the heme in human blood, or that the consumption of blood might ease the symptoms of porphyria, are based on a severe misunderstanding of the disease. There is no real evidence to suggest that porphyria had anything to do with the development of the original folklore, as the hypothesis is mainly based off the characteristics of the modern vampire in any case. Others argue that there is a relationship between vampirism and rabies, since people suffering from this disease would avoid sunlight and looking into mirrors and would froth at the mouth. This froth could sometimes look like blood, being red in colour. However, like porphyria, there is little evidence to prove any links between vampires and rabies.
I hope this answers your question.