This question has been answered by many people who have given great answers to this question, still, this continues to be on the mind of many young awakening vampires and even some of the not-so-young ones.
We will start by defining vampirism as a condition. Vampirism is a physiological condition in which the body does not produce enough energy or life force to carry on with daily activities.
Those without vampirism can produce enough energy for daily activities by eating and resting well. For those with vampirism this is just not enough; I will point out that this doesn’t mean that it’s not necessary, we still must eat and rest well, we just have to get some extra energies from other sources.
Now that vampirism is defined, we’ll take a look at the most common energy sources we can “feed” from. Vampirism tends to be classified by the sources we take our energy from.
Psy - This is, as far as I know, the most common form of vampirism. Energy released through daily activities, energy released when there is physical or even mental strain. This energy has been known to be abundant at places were many people gather, such as malls, schools and other public places where people are just giving off psy energy to the ambient making it easy to assimilate.
Empathic - This is the energy given off by feelings. Those who feed on empathic energies tend to develop a link between their donor and themselves, the stronger the link, the easier it is to feed off this kind of energy.
Elemental - This is the energy that comes from the elements: air, water, fire, earth. I once read an article by Belfazaar Ashantison about the “ecopath” this is a personal theory of his, but I would back it up with my own experience. Basically, elemental vampires tend to be more attuned to their environment, so if there are changes in it, they will feel them. For more information, look for the article in the VC magazine files or contact me, I’ll be happy to explain further or give you the link to the article.
Pranic - This most commonly refers to the energy that can be acquired through sexual relations. There is a lot of energy in sexual relations and this energy can be assimilated by those who feed pranically.
Sanguine - This is the energy contained within blood. This, however, is not as common as many would think. Those who only feed sanguinary are, as far as I know, quite rare for most feed on the other types of energies.
There are other known sources, but these are the basic ones; personally, I think others such as negative or chaotic are sort of branches from the basics. This is however only my personal opinion.
Now that we know the basic types of energy, we can talk about “adaptive vampirism” and “eclectic vampirism”.
Adaptive vampirism.- This is when there is a need to feed from a certain type of energy, but that type of energy can be substituted by another type if the needed one isn’t available. As example: If there is someone who has a need to feed empathically, but he finds no sources of this energy, he can take energy from another source like elemental, until the needed energy is available again. Those who are adaptive can feed from anywhere from two to all types of energies. There are those however who can only feed from one source.
Eclectic vampirism - This is when there is a NEED to feed from all types of energies. An eclectic vampire must struggle to maintain a balance of all energy types, which can be a difficult task, since availability of every type of energy varies with every situation.