Dimitru Staniloae, the great Romanian Theologian, offers an excellent introduction to the passions as understood in Orthodox Christianity. The following excerpt is from his Orthodox Spirituality, which I highly recommend.
The passions represent the lowest level to which human nature can fall. Both their Greek name, pathi, as well as the Latin, passiones, show that man is brought by them to a state of passivity, of slavery. In fact, they overcome the will, so that the man of the passions is no longer a man of will; we say that he is a man ruled, enslaved, carried along by the passions.
Another characteristic of the passions is that in the man unquenchable thirst is manifested, which seeks to be quenched and can’t be. Blondel says that they represent man’s thirst for the infinite, turned in a direction in which they can’t find their satisfaction. Dostoevsky has a similar idea.
Neilosthe ascetic writes that the stomach, by gluttony, becomes a sea impossible to fill – a good description of any passion. This always unsatisfied infinity is due both to the passion in itself, as well as to the object with which it seeks satisfaction. The objects which the passions look for can’t satisfy them because objects are finite and as such don’t correspond to the unlimited thirst of the passions. Or as St. Maximus puts it, the passionate person finds himself in a continuous preoccupation with nothing; he tries to appease his infinite thirst with the nothingness of his passions, and the objects which he is gobbling up become nothing, by their very nature. In fact, a passion by its very nature searches for objects, and it seeks them only because they can be completely under the control of the ego, and at its mercy. But objects by nature are finite, both as sources of satisfaction and in regard to duration; they pass easily into nonexistence, by consumption. Even when the passion also needs the human person in order to be satisfied, it likewise reduces him or her to an object, or sees and uses only the objective side; the unfathomable depths hidden in the subjective side escape him.
Staniloae goes on to detail the workings of the passions, both the natural (those rightly rooted in our nature but now misdirected) and those that are unnatural. Thus we have a natural passion (desire) for food that is right and proper. We are, after all, biological creatures. But we are also spiritual beings, and when the desire for the infinite, rightly directed to God, becomes confused with the natural desires we wind up with unnatural passions, such as gluttony, etc., in which we desire infinitely what should only be desired in a finite and helpful manner.
Thus, in short summary, the passions are the energies or desires of our soul or body that have at their root a right and proper end, but, because of the fall, they are disordered and are misdirected seeking after what they can never have. As such, they are not to be confused with the emotions, per se, though the emotions, too have a proper role, and can be distorted into a passionate and incorrect state.