Desires: Beliefs about one's needs and what one should like to happen to one's self and others. Fundamental propositional attitude.
As I use terms, desires are to some extent, at least, reasoned out, and usually depend in part on one's beliefs about the situation one is in and what is and is not possible in it; on one's further desires, and how one has ordered these in terms of priorities; and on what one believes about oneself.
The basic difference between needs and feelings on the one hand and desires on the other, as I use these terms, is that needs and feelings are given to one in the manner of sensations: if conscious, one has them, and they inform one in various ways about the states of one's body, whereas desires concern ways and means to cope with one's needs and feelings, given one's beliefs, and may include much more about the state of the world or of other people.
That is, as soon as one has anything like a real desire one has both a value, that refers to one's feelings and expectations, and one has something like beliefs about whether the desire can be satisfied, and if so how, and if not why not (here and now).
Thus desires together with beliefs enable human beings to design plans and policies to cope with their needs and feelings.
Here is Spinoza on the subject, from Part III of The Ethics:
I. Desire is the actual essence of man, in so far as it is conceived, as determined to a particular activity by some given modification of itself. Explanation. - We have said above (..) that desire is appetite, with consciousness thereof; further, that appetite is the essence of man, in so far as it is determined to acting a way tending to promote its own persistence.
This is neither very cogent nor very clear, but it is to the point: What human beings consciously do, at least, to a large extent depends on their own desires, including their fears, and these desires are formulations of ends that relate to a human beings' needs and ideals about what the world, others and oneself should be, according to oneself and ones own beliefs and values.
And here is Buddha on the subject, from the Dhamma-Kakka-Pravattana-Sutta, as rendered by Rhys Davids in Buddhist Suttas:
Now this (..) is the noble truth about suffering.
Birth is attended with pain, decay is painful, disease is painful, death is painful. Union with the unpleasant is painful, painful is the separation from the pleasant; and any craving that is unsatisfied, that too is painful. In brief, (..) the conditions of individuality and their cause are painful.
Now this (..) is the noble truth concerning the origin of suffering.
Verily, it is that thirst (or craving), causing the renewal of existence, accompanied by sensual delight, seeking satisfaction, now here, now there - that is to say, the craving for the gratification of the passions, or the craving for (a future) life, or the craving for success (in the present life).
Thus, Buddha and Spinoza agree on the centrality of desires for human beings, but Buddha seems more radical in holding - rather paradoxically - that suffering depends on desires, that the condition of human beings is disposed more to suffering than to satisfaction, and that therefore it is recommended to give up desires so as to give up suffering.