I would say that Jhanas are the final stages of the progressive development of the state of inner harmony, that is the Buddhist path.
The entire Buddhist path is based on one principle, that psychological discomfort comes from inner conflict and that therefore eliminating inner conflict leads to inner harmony. Secondarily, outer conflict leads to inner conflict, and ethical behavior is defined as that which prevents outer conflict.
The prerequisite to Jhanas, therefore, is to eliminate all coarse conflicts that serve as obstacles to the attainment of harmony, such as external entanglements and various real life sources of personal drama and regret. Hence the emphasis on ethics, non-attachment, and non-egoism for beginners.
Once the life of an individual is more or less peaceful on a day to day basis, the student can focus solely on their mind and attain Complete Harmony. This is the role of Jhanas.
Jhanas are generally divided into two stages. On the first stage, student uses emotional intelligence techniques to generate a state of happiness. On the second stage, the student contemplates the way things work and perfects the state of harmony, culminating in what's known as suchness. The premise here is that harmony is a refinement of happiness, and suchness is perfectly refined harmony.
The particular techniques used to generate happiness vary between schools. In early Buddhism, the main technique was to review one's own ethical purity, one's philosophical realizations (insights), and one's peaceful lifestyle, comparing that with regular people, and by congratulating oneself on actually measuring up very well, generate the sought state of euphoria.
In conservative descendants (Theravada) this technique was preserved as part of the so-called anussati (recollection meditation), but divorced from the practice of jhana proper. In fact Silanussati, Caganussati, Buddhanussati, Dhammanussati, and Devatanussati are perfect themes for the first jhana. See e.g. this explanation for details on each. Since connection between positive recollection and the first jhana has been lost in Theravada, for them jhana became an exercise in brute-force one-pointed concentration, which eventually leads to a similar state of joy but requires much more time and effort.
In Tibetan schools, the technique was largely retained but shifted from recalling ones actual achievements, to substituting one's self-image with that of a deity ("yidam"). I imagine this so-called "generation-stage meditation" to be a development of the Devatanussati technique. This is thought to have a better effect, since the imaginary self has no limits to its perfect qualities and can be optimized to counteract particular student's emotional hangups. Naturally, this goes at the expense of no longer having one's ethical achievements be the necessary basis of meditation, which Tibetan schools compensate by greatly emphasizing compassion.
The second phase of jhanas is perfection of harmony. This is when the student realizes that the contrived state of happiness cannot be perfectly sustainable and instead learns to accept the natural uncontrived moment by moment state as the basis for the absence of inner conflict. This deepest acceptance of whatever arises at every moment, inwardly and outwardly, is known as suchness and is the summum bonum of Buddhist path.
Again, the original technique underwent mutation in historical schools. In Theravada they say that upon contemplating Three Marks of Existence seeing impermanence of all contrived states and inevitably of occasional suffering, the student must go through catharsis of accepting the existential failure, and emerge having completely lost all grasping to any experience. Reference "Sixteen Stages of Insight".
In most of the meditating Mahayana (such as various types of Chan/Zen), a state of non-judgmental awareness (don't accept, don't reject, just watch) is practiced as a straightforward way of cultivating suchness (following the beginner's breath meditation, therefore skipping the generation phase), but it's place in the big scheme of things is not usually explained, other than mysteriously stating that this is the very state of the Buddha.
Finally, in Tibetan schools, this phase is practiced in conjunction with study of emptiness. For philosophically inclined students, analytical meditation on emptiness serves to dispell doubts that any contrived state cannot be It. What follows from that is staying in a wordless realization of this profound groundlessness, which serves as an entry into suchness. For students of less intellectual bent, the practice takes form of so called meditation of no-meditation, which is a name given to cultivation of appreciation of suchness as the primordially natural state.
In any case, the path usually starts with removal of coarse obstacles, following by cultivation of the seemingly perfectly conflictless state, followed by realization that any contrived state is inherently unsustainable and conflict-ridden, followed by contemplation of the implications of this realization, followed by direct cultivation of suchness and complete liberation. The Jhanas symbolically refer to the non-preliminary stages of this sequence.
I'm necessarily glossing over many nuances and simplifying a bit, but I think this covers most of what you asked, let me know if you have further questions.
